Because most of what they kill off are things like the desktop process which is useless while a game is running unless you want to alt-tab to it, look through explorer, see the useful UI's, etc.
It's not bloat relative to regular computing needs, it's bloat relative to having a game running in full screen.
And you CAN optimize your system for gaming and "de-bloat it" it's called killing processes unnecessary for gaming, you just can't kill the main desktop process... which is needed for like... using it as a desktop.
And killing processes unnecessary for gaming could otherwise slow things down from a general desktop user standpoint. Having free memory is not what makes software faster, USING memory is what makes software faster... hence why having more of it free for gaming can make a game run faster because the games can then USE IT, they could cache more assets for instance. Same thing is true for Windows, it using memory might be speeding some things up (like launching explorer, Edge, the start menu, etc.)
There's also random "maintenance" processes that won't launch in FSE that eventually NEED to be launched to have have a hosed up desktop experience.