I think with celebrities we associate a god/goddess-like status upon them much the same as we do comic book characters or other fandom characters. Yes, they are real people, but as a society we've built them up to be beyond human. They are our modern Greek gods. We wish we could be in their presence, delight in their triumphs, and disgustingly revel in their defeats. They are flawed, but perceived to be "perfect" or at least greater than us mortals. Sports fans wear copies of athlete's work clothes (jerseys), but no celebrity or athlete is dressing up like Steve the accountant or Sarah the doctor to be like, "Yeah, go Sarah! She's the GOAT!"
Bringing it back to erotica, no one really cares about Laura, your MILF neighbor, because she's essentially no one to the rest of us who don't know her, so whether she's your real neighbor or a made up character, it doesn't matter as much to the reader as it does the writer. But Laura the celebrity is someone we can all picture. We've seen her body of work and likely a lot of her actual body. She's not attainable (though one could argue if Laura the MILF neighbor is attainable but she is next-door and likely within your socioeconomic sphere), so a story about Celebrity Laura is fantastical, much like a fictional character, and we dismiss that she's human and someone else's neighbor, friend, sister, mother, etc.
Plus, often what we know of celebrities is much like the aforementioned Greek stories or fictional characters, a crafted narrative or persona. Even if we read their biography and watch every interview, none of us really know that person. Yet we all feel we do to some degree, at least to the point of being able to easily immerse ourselves into a story (erotic or not) about them and not have to delve too deep into characterization or physical description.