As mentioned it is a mountain of work to do a port. Generally speaking code when it is compiled (which the PS1 often was) does not go back to being code (see stuff like the halting problem), and you lose a bit of data in the process. This is getting to be less true as time goes on -- there are little things that you can observe that speed things up, as such decompilation as it is called is right at the forefront of computer science research (as in I can easily see people doing it for phd research projects for many years yet, and such places are often where we get the advances in it) but we are nowhere near anything like a one click solution and may never be (outside of serious AI anyway). The PS1's hardware is also a less common target for such efforts, though as noted above the Mario 64 stuff happened and the N64 is similarly not popular hardware to look at.
Technically you don't need to decompile it either as you can figure out all the mechanics of the game (for the mario 64 stuff that is every move you can do (including water, flying), how the controls affect said moves, every collision/hit/interaction with an object, every object behaviour, every enemy behaviour, and how all the levels work, music works, graphics work, every animation and so on and so on) and replicate that in something else. Nobody really does that for PS1 era games though -- it is not so bad for an old arcade game, C64 game or something but for most things in the PS1 era where go anywhere, do anything and anything can happen at any point then yeah. People tend to reserve such efforts for things like document formats, and even then it is usually incomplete (see the fun and games opening microsoft office formats in things that are not from Microsoft, and indeed things that are from microsoft but several versions down the line).
If you are aiming for a PC or something then it would be far quicker to go with emulation. If you are all about one game then you can probably do all the fun high res texture replacement, figure out what widescreen needs for this and maybe kick controls to something more pleasant than a control pad if needs be.