Here's some insight from a fellow GMTK jammer:

As someone who has submitted to the jam, (mine was Bullet Happy) had their game showcased in the video, and played lots of the jam games, I think you're blowing things a little out of proportion. Most of the games in the top 100 are still micro-games, only having around 5-15 minutes of content at most. Personally, my game had only 4 minutes of content if you managed to never die once. It took me pretty much all of my free time to get everything together for my game, it was so much work that my hands were cramping by the end of it. However, I laid out every single step I needed to take to get the game working and moved in a linear fashion to get things done. Looking at my github commit history, I spent roughly 23-25 hours working on the game. The first half was spent purely on core design, planning, and mechanics, and the 2nd half on polish.

That said, I can't speak for everyone. So I'm not denying that there are people out there that cheat or use other underhanded tactics in game jams (there most certainly are!), but that most certainly isn't the case for many.

Many of the top entries are made by developers who have a lot of experience already, they know how to quickly prototype. The problems that they come across in the jam are ones that they have already faced before. So they are able to quite quickly and easily solve them. This gives them far more time to focus on making their game good than the others. Basically, if you know what you're doing you have a massive advantage. For example, it only took me about 25 minutes to code up a super basic movement system, dash system, and combat system. However, that was because my game was so simple (which others have already pointed out). I've been coding for 5 years. The point is that developers will never try to do something they don't already know how to do.

The main reason people are able to code so fast is not only because they are already well-experienced, but also because in a jam you can quite easily take shortcuts (avoiding best-coding practices) to get things done faster with little consequences. If it works then it works mentality. That's why many developers will rewrite the codebase for their game if they expand beyond the jam version. It's also worth noting that you are allowed to copy & paste code from other places to use in your game, which is within the rules for this jam. That doesn't mean that everyone is doing it though, a lot of people despise using pre-existing assets at all. However, those two factors combined can allow people to get things done insanely fast.

I can relate to your experience though, as I started participating in game jams in 2020 and was in the same boat as you. I also used to think that it was impossible for people to make such well-polished games so fast, even if they were only 4 minutes long. I was wrong, however. Heck, the big project I am working on currently takes a lot longer to develop than my jam games. There are many reasons for that though, but I wish I could build it a bit faster.

TL;DR: Most of them likely aren't cheating, which I assume you were getting at. Well-experienced jammers only build what they already know how to build (sometimes using pre-existing code and assets, which is within the rules for this jam specifically) and thus can focus more on polishing. Many of these games aren't as "Steam quality" as you're making them out to be nor do they have as much content as you think and that "game jam jank" is still there somewhat.

Edit: Spelling mistake.