Because it's actually dogshit. Like in every thinkable way a launcher "could be bad" it is.

Examples:

-Cannot move games or files, IN ANY CAPACITY. If you move a game folder or file the Epic launcher loses sight of it and the launcher has no way at all to tell it where existing files are. I learned this when I tried to move GTA V and had to reinstall the full game in the exact same location so that epic could see it.

-The launcher is the slowest loading launcher and service in the world. I have a 7800x3D and an nvme and EGS is the slowest launcher on my computer by a country mile (fucking Uplay is faster). Also on top of that, it has a major hard on for making you log back in on the same fucking computer (what feels like weekly). Meanwhile I don't touch steam for 2 weeks and, guess what? It still logs me in! How the hell did they figure out that crazy tech?!

-It has absolutely 0 of the function the steam launcher has. Besides letting you spend money on games and launch them. No communities, workshop, friends features, profiles, voice calling, steam share, remote play together, etc.

I could go on but this all just grinds my gears when they do nothing but tout how they are "for gamers" and "for developers" when they're clearly just here for fucking money. They use anti consumer practices to lock people into an ecosystem that for some reason they refuse to improve (wild fucking concept, maybe people would use their launcher if it wasn't one of the least functional ones available!), and instead try to bait people in and keep them around with a free game a week. I'll never, ever willingly give a cent to epic games. They've proven they don't give a fuck about gaming or the consumer experience

edit: changed you to they when referring to Epic Games at the end