Because it means they are going out of their way to unlocks things they don't need to unlock. That implies there's something deficient about the first days of the gameplay experience. As in, the "infinite" things they could do are instead insufficient.
Yeah, you can argue you don't need to not cheat, but it's circular logic: you don't need to cheat, either. The only explanation I can muster is that people succumb to temptation and impatience. Okay fine. But then people are going to point that out. Especially when that temptation and impatience can have spillover effects into others' gameplay experience, as I explained earlier.
Let me clarify something here: You mention stuff like going to other islands to get all the materials you get, and tricks to get infinite Nook Miles and stuff. In other words, you're focusing on possibilities and being as efficient as possible.
Those are things that are technically possible, yes, but are things that are not only not necessarily intuitive or obvious, but not necessarily a fun or interesting way to play for a given person. It may technically be possible, but that doesn't mean someone actually wants to do that, or there's anything wrong with that.
Like me for instance. I don't have New Horizons like I said, but I have no interest in like the crafting system in general or gathering materials by shaking trees or breaking down rocks and the like. Like, the crafting system such as it was wasn't very interesting in Pocket Camp, and it doesn't seem very interesting in New Horizons to me either. So if I were to pick up the game, I'd very likely try to get by on the absolute minimum number of materials on any given day. Doesn't matter that I can technically pick up more, that it would be more efficient for me to do so and save me time in the long run or anything like that, when I just don't find that type of gameplay interesting.
Like, it might be technically possible for me to do that stuff without time traveling, and in a more efficient manner at that, but that doesn't really matter to me at all if that more efficient gameplay style would be much less fun to me.
Or to put it a completely different way, I mentioned that I personally recognize these games as having infinite potential, infinite avenues for creative expression and all that. Like, I gave the example in my first post of this thread of say making a Pokémon themed town, and alternating between different themes based around different types of Pokémon, different sets of Gym Leaders, different Elite Four Members, different evil teams, or even individual Pokémon, and how there's tons of other things you can do with just a simple theme like Pokémon to keep things fresh.
But yet at the same time, in my previous post in the thread, recall that I also mentioned that there are those in thread saying that for them personally, they don't like the idea of time travel because to them, if they were to time travel, they feel that would exhaust the amount of content the game has to offer way to quickly. I mentioned how I personally don't agree with those that say that, how I don't think that's how the game personally works to me, because to me, the game has infinite potential for creative expression, but yet despite that, DESPITE that, I can still totally respect the people who say that anyway, despite I myself seeing the game to have infinite possibilities like that.
How can that be, that I can still very easily see that point of view, even if I disagree with it?
Well, because that's the thing: that's because in my example, I'm taking about how I'd play the game and the type of things I'd like to do if I were to play it.
However, what I personally like is not the same as what SOMEONE ELSE likes. Like, to go back to my example, I mentioned the example of constantly changing up my town a lot and changing my furniture and such around by keeping a central theme as a basis but changing what aspect of the town I want to explore.
But someone else? Someone else might have a very specific idea of what they want their house and town to look like, and once they reach it, that's just how they want to keep it, and they have no interest in making any more changes to it once they get there. The fact they technically CAN keep changing it means nothing at all if they aren't interested in that or don't find it fun at all. It needs to not only just be possible, but also be fun and enjoyable to matter. And so like if that's something that someone's not interested in, that's obviously also the type of thing that would limit how much they personally can get out of a game like this, and thus how they personally could easily think time travel could limit how much they can get out of the game, whereas someone like me wouldn't be bothered. Just complete differences in gameplay style like that.
And obviously that's just as valid a way of playing, having an end-goal of what you want your island and home to look like and not wanting to deviate from it once you get there, as mine of wanting to constantly mix things up, but yeah, depending on which one you pick, that could quite easily affect the longevity of the game in multiple ways and thus how people view time travel and stuff as well.
For your example of going to different islands and use Nook Miles and stuff to get infinite materials and stuff, that same type of thing applies to me. That might technically be possible... But it doesn't matter if that's not a fun gameplay style. Both need to be true for it to matter, not just one or the other. I hope that makes sense, especially with that other example to show what I'm trying to get at there. That you seem to be getting stuck on what are technically possible ways people could possibly play the game yes, but aren't necessarily ways people would enjoy playing the game despite it technically being possible and how those are two very, very different things, and can affect the ways people play and enjoy the game in turn due to that. That possible isn't all there is to it. Hopefully that makes sense.
But yeah, for me, it all just comes down to different gameplay styles like that, and people just having to personally find what works out best for them personally, with their being no right or wrong answers but people just having to find their personal answer that finds their personal gameplay style and as long as that what happens, that's all good, and that's all that matters to me as far as these things go, anyway.