This gives me an even worse headache than playing a fictional character that's relatively well known with hundreds of theories and quirks I could possibly use to get into their heads. I distinguish playing existing characters into categories, and playing real people goes right alongside playing someone's invested small-scale original character. I cannot and will not do that.

I've only seen this in contexts where literacy and/or plot quality is stunningly lacking. Otherwise, though, to get into a head like that requires a level of intimate knowledge that I simply don't want to have with any particular person. The only people I would know to that extent are such close family that they have no purpose in a roleplay of mine.

To play it at the literacy and knowledge required to simply not care of the many presentation flaws on both sides is to do something I would be highly unwilling to humor. To play at the literacy and knowledge required for accurate representation strikes me as creepy, unnecessary for my reading, and too much time just to get to this point.

As for 'why', I'd imagine it's a self insert fantasy and/or "I want to see this" fantasy with few roots in plot and character most of the time, and simply a relief for one's desire to see that scenario happen in one of the only ways that it possibly could. Most of the times this does end up being smutty, but it doesn't have to be this way either.