Speaking at least for myself, if I step away from any code for any length of time (even a day), then when I get back to it I pretty much immediately see and feel how gross it is. There's never been an exception to that, other than maybe for things as "trivial" as "alias ls='ls -FC'" in my .bashrc. (But even there of course there's a zillion ux problems to be unpacked and talked about even if they don't manifest themselves right there and then.)

Why is code mostly so "bad"?

Why does it take much effort to "swap in" what you need to know / prevent yourself from feeling about the code when you come back to it?