...why not immerse oneself in a First Person view ...
What is immersive to you is not necessarily immersive to me, or anybody else.
I can'tt play in 1st person. If I try to fight in 1st person, I die. If I try to move around in 1st person, I walk off cliffs and into furniture.
I generally play characters that are very different from me. So seeing that character on the screen reminds me to not play as myself (which is not roleplaying as far as I'm concerned).
Playing in 3rd person also matches the
style of roleplaying I do. I do not inhabit my character's body. I am a kind of 'presence' hovering over the character I'm roleplaying. We're a party of two, my character and I.
In addition to all that I don't like having to look at the hands of my character holding a weapon directly in front of them, at arm's length, in 1st person. I can't imagine running that way, it looks awkward to me. I also don't like having to look at a crosshair everywhere I turn. It breaks my immersion to have to constantly turn it off and it breaks my immersion to have it turned on.