So knowing that "it's complicated" responses are generally sh*tty, I'll nevertheless respond with, "it's complicated." Fundamentally, there's a very different core UX expectation with a controller. First off, you're limited to 16 buttons. And the core expectation around how those buttons work is quite different from MKB. With a mouse, there's an expectation that "clicking on a thing" will do something. Controller logic in this regard is quite different; we try to avoid having players hover-over-a-thing-and-press-a-button. Now, sometimes, that's unavoidable (e.g. deathbox looting), but it's generally not great. In most places, we try to make it so that you see that a button - e.g. X - is bound to some action, and when you press X - anywhere - something happens. Now, this leads to some weird behaviors too because of core aspects of our UI framework (and trying to support both controller and MKB simultaneously and fairly seamlessly) where hovering over a "button" (on screen) and pressing a controller button results in some seemingly unintuitive behavior.
We do allow some remapping, but there are core actions that we NEED to rely on - that mouse clicks take care of with MKB - that we basically fix to X, B, & A (using Xbox as a reference, because that's what I play with and because that's natively supported on PC).
Now, everyone has their own opinion about how we solve some of this stuff. Like moving-and-looting. And those ideas all make sense. The problem is that there is no single idea, and many ideas are in direct contradiction with each other. So, of course, the logical response to that is, "just allow full customization!" which is a great idea, except it's really hard to do well.
Even small changes can have big knock-on effects. I did an internal rework of the deathbox menu (and Loba's ult that shares a lot of the same logic). I ended up breaking it for people who mapped their "A" button to alt-use (because who does that?). I was able to fix it, but the fix also FORCED the "A" button to behave the same for everyone.
I know these answers can feel like a copout. "It's hard" is definitely not a reason to not do something. But this is like really hard. Especially to do in a way that isn't so totally arcane esoteric, especially since the vast majority of players never, ever touch this. Or want to touch it. So it's high risk technically. With a very limited engagement of players. And even the most complex solution is unlikely to please everyone. So we try to focus on the really core interactions - e.g. the reload/res or res/door open problems - and how we might solve those. And those areas where there's pretty overwhelming feedback - like the survival slot and inspect issue. But it's a huge leap from those things to "I want to be able to make my controller do whatever I want" without having those changes break the game or be so cumbersome that maintaining them and handling them for every future feature is, speaking in broad terms, simply impossible.
Development necessarily comes with tradeoffs. That stinks. But it's also just reality. And fully customizable remapping is just not possible. If you really, really want that level of remapping, it's almost certainly going to come at the hardware level - e.g. something like the XBox Elite controller, where you're forcing the controller itself to send different outputs in response to certain inputs.