I play mostly in third, but I like having the option to use first-person for examining things up close. I like being able to see my character in the world to get a better of sense of physicality - first-person always feels unnatural to me (I get why people prefer it but to me it feels like my character has tunnel vision and walks around with their hands held in uncomfortable positions.)
I have seen first-person games that put your feet in the game, I think even back around 2000-2001 or so there was a Jurassic Park game that modeled your entire character's body, so when you looked down you saw your torso, etc. Technology-wise there's no major hurdles to doing that in a game and as far as resources it's just one more character model so nothing terribly drastic there. My own 3D training is about a decade out of date (and I wasn't very good at it anyway - it was part of my minor, though,) but I can take a couple of "educated guesses" as to why you don't usually see your feet/body in first-person games.
For starters there's some gameplay considerations. In practice, if you're looking at the ground it's because there's something down there you want to look at - this can be problematic if your feet get in the way of your view.
I have seen games that render your character's body and place the camera in the head, but this leads to issues too (not insurmountable difficulty, but time consuming and tedious, surely.) For one, players have a sense now of how things "should" look in a first-person game, and that doesn't correlate with real human posture. You have to animate the body so your hands are always held at about shoulder height, for instance. You also need to make some tweaks to FOV and rendering so things don't look out of proportion that close to the camera. To really do it "correctly" and have it animate so that it looks naturally you'd actually have to build out a character model that's somewhat out of proportion.
Of course in first-person games you're really just seeing a model of the player's forearm that "floats" just under the camera; when you look down and see feet you're really looking at two models of disembodied feet. Makes things a lot easier to animate and render. The reason you don't see it very often is likely because most devs just don't feel it's worth the effort I'd imagine. There's no technological reason it can't be done every time, but it would take extra effort to make it look "right" and not be a potential annoyance in game-play ("I really want to look at this thing on the ground, but I can't because my feet get in the way.")