First of all, because space is limited. You don’t want your laptop to be too much big, after all, the purpose of laptops are that they are portable and small. Also, there would be heat problems with it, you can’t just put anything you want in it, it would be hot as hell.. You can’t add fans to it as you can to pc, because the casing is very,very small and limited, so it’s not easy to make. Also, you wouldn’t find components anywhere, because every single component in the laptop is specifically made for that laptop.
If it’s for gaming, buy an PC. After all, PC’s are there for that.
Laptops aren’t for gaming. Yeah, there are gaming laptops, but they can’t compare to gaming pc’s. Also, gaming laptop will cost you even more same or even better pc.