The first person RPGs I can think of are :
- Arx Fatalis
- Deus Ex series
- Elder Scrolls series
- newer Fallout series
- Might and Magic series
- Vampire the Masquerade: Bloodlines
Not a lot but certainly some considering these are rather well known games. Maybe M&M and Arx Fatalis are the least known of these but still - first person RPG is not unheard of and indeed a lot of people would know of them at the very least.
As for why there aren't more: I think it's "traditional" for RPGs to be in third person in some fashion. Be that more of a top down view like Infinity Engine RPGs or actual third person view like, for example, Gothic. I suspect it a mix of historical reasons that drive that - cRPGs started off very close to TT RPGs but with the computer doing most of the calculations while you just played. So, naturally, it'd play similar to how a TT RPG would play - using a sort of a battle map and figures to represent the characters. That's mostly the easiest way to represent these types of games. And lack of good video options did also contribute here - drawing a first person view would have been way more work than an enhanced chess.
So, this is how they started off. Technology developed but the foundations had been laid out. It also meant that the traditional combat was tactical and turn-based, so, again - not that easy to represent in first person (though M&M does have that option, actually).
Later on, with better tech, we could do away with turn-based combat, as there was sufficient power to run battle simulations in real time. However, again, turn-based was traditional, so some games kept it. Arcanum Of Steamworks and Magick Obscura allows switching between both real time and turn based combat to suit either playstyle. At any rate, the turn-based combat came from a different tradition and that was to have the player also recruit a party, so usually you'd play with more than one character. You can't control the party in real time, not really easily, hence why turn based RPGs tended to persist.
When real-time started to become a standard, one way of "fixing" the party members issue is to remove them. Or at least limit them to one or two guys. Still, with more action-y gameplay the top down view was not as good, however, to keep with the spirit of it third person over the shoulder camera was chosen as a good perspective. It also helped with seeing your character and their equipment, which was also one of the traditional draws of RPGs - equipping your toon. Getting a new pair of green pants and then the character on screen changing their pants was part of the expectation. Well, depending on the game, they might only change some equipment but the point stands - you'd start off with nothing but rags (sometimes literally) and eventually you'd have demon/dragon/whatever slaying badass draped in shiny armour.
Sure, some games did break out of this and went first person - and they were actually recognised for it. They did show that the genre can exist that way. Still, many developers heed the call of "tradition" and strive to make game similar to many an RPG.
Would that cycle break? Probably not. Or not soon-ish. Still, at least some developers would be bringing something new to the table. Maybe not soon but it's bound to happen.