Hell, I'd love to hear from a developer who could offer some insight from their side of the equation in this thread.
Sure thing.
1. Most indies (myself included) barely know what they're doing so it's easy to make stupid mistakes.
2. Most game developers develop and test on good PCs. If you don't test your game early on in development on your weakest target hardware, you can discover late in development that your game just won't work there, requiring lots of heavy reworking. At this point, many developers don't have the money to do the time-consuming work and just do the bare minimum to get the thing to run. Personally, I ran into this problem with Cosmic Star Heroine - when I first tried to run it on our Vita devkit, the game almost immediately crashed. It took me months to get it working on the devkit and months after that to get it working on actual Vita hardware (the devkit is beefed up compared to actual consume hardware). As a nice side effect, since I managed to get it to run okay on the Vita, performance is excellent on the much more powerful Switch.
3. Games are frequently worked on until the last minute. A last-minute fix for one problem often causes problems elsewhere that aren't realized until someone spots them.
4. There's a lot of bureacracy involved in getting a patch approved on consoles so even if you have a fix, it could take days or even weeks before you can actually give it to players.
5. Your average indie dev teams' playtesting/debugging resources are themselves. Like with our last game (a 10-16 hour JRPG), I think only one of my friends / fellow devs actually played through the whole thing before launch. Most of them played a couple hours and then told me they'd rather wait for the finished version. Nobody was playing the game multiple times on different versions. Basically, Bill and I had to play the game over and over. As only two people, we were going to miss stuff. And at some point, you're so sick of playing the game that you just want to ship the thing.
6. Games are far bigger and more complex under the hood now than they were in days long past. Because of this, debugging is much more difficult and there's only so much that more time and money can do.
7. In the end, the number of people playing a released game is generally going to be exponentially bigger than the number of people who worked on it or tested it. A million people playing a game are going to find bugs that dozens of playtesters missed.
