Why? I don't really know. I just know I don't enjoy it, and don't read books written this way because of it. It's boring to read "I did this, I did that" for three hundred some odd pages. It sounds as if the author/protagonist is blowing his/her own horn all the time with the ad nauseum chorus of "I, I, I."
First person is probably the easiest way to write a book, as little thought has to be given to the inner workings of the minds of the characters other than that of the narrator. Why? Because the narrator cannot GET into the minds of the other characters -- he or she has no way of doing this -- so the story must be told from only ONE point of view. This makes the writing easy, and avoids what some critics view as the sin of writing from multiple POVs in one story, but to me, it also makes the story boring and one-dimensional. I want to know what the other characters in the story are REALLY THINKING -- the author's conception of what their real internal feelings are -- not what the protagonist THINKS they are thinking. There's a big difference there. It's much harder to do, but so much more interesting and informative when it's done.
Call me a snob, a blockhead, whatever. Some members of a group I am in here must think of me this way because I do not participate in all the group reads, but this is the reason. Too many authors these days, especially indie authors, are writing in the first person, and it is just not worth it to me to spend time reading something I don't enjoy when I could be reading something I do. In any art form, and that includes the art of writing fiction, beauty is in the eye of the beholder. I like what I like, and first person is NOT IT.