Because logically, yes, everyone knows that Pokemon, the single largest media franchise on planet earth, is going to sell very well barring very extreme circumstances. Emotionally though people can't help but be upset because a game franchise they care about, in their opinion, is going downhill and nobody else seems to care. which means there's no reason to ever make it anything other than what it is now. Which, in my opinion as a player of these games for nearly my whole life, is that they are fun for sure, I won't deny that at all, but also kinda mediocre. The franchise hasn't changed much in any meaningful way since its inception beyond adding a bit more plot elements and transitioning to 3D graphics, and in this iteration adding semi-open world elements (that in the end don't really change the core gameplay as much as I would have hoped, but is still a welcome addition). For new players, or fans of the franchise that have skipped a few games, or even core fans that were soured by Gen 7 with its near oppressive dialogue and the like, they definitely would seem fresh and an improvement. I can't deny that there are definitely aspects that have been improved, but also some have gone backwards, such as Y-Comm being a kind of shitty online system, and the cutting of over half the Pokedex. I totally understand why people like these games and why they would sell so well, but it does have implications for the series going forward that these games could sell so well in spite of so many complaints (albeit seemingly from a vast, vocal minority) those implications being for better or worse. In the end though, games evolve over time, and sometimes they evolve in such a way that may be distasteful to you. At a certain point one needs to accept that criticism isn't going to change the course of that evolution, and accept that you just aren't the target for these games anymore. I don't think we've reached that point in the cycle yet, though.
Anyways, I don't think the snarkiness or occasional aggressive attitude is necessary, but i understand why people are feeling down about it. I think those opinions can and should be voiced, however not in such a way that belittles others (same for people who have positive opinions about the games tbh). It's just a video game after all.