Why are game store clients shit?

I marvel at the inability of huge publishers to build what should be a simple app. Sure, taken as a whole, what a store does is pretty complex, but it can be broken down into smaller problems, each of which has been solved already. Those problems were not easy to solve, it took thousands of hours of work from talented and smart people. Why do you try and fail to solve them yourself?

Qbittorrent was developed by TONY STARK IN A CAVE USING A BOX OF SCRAPS, and it does what EpicGames cannot and what is janky with Steam. It can resume downloads. It can discover existing downloads. I do not need to worry that if I interrupt an 80GB download which over the course of half a day has already done 30GB, I will have to start over.

Aria2 is also a thing that exists. Are these technologies not in use because of copyleft licenses? Do companies believe they can build something better with the army of cheap React devs they throw at their electron apps?

How is their stuff more janky than what a few volunteers have built?

And I know that EpicGames thinks their client is a stable and performant piece of software which nobody else is able to build and their secret sauce which gives them a MaRkEt AdVaNtAgE, but if they were to open source it, would anything change? People buy games from them, they care about the games, not about the program which downloads them. Epic sells games, not electron apps.

Wouldn’t the ability to deeply integrate a working downloader make that worth it?