What the title says! OK, so this has been a problem ever since getting Late Night back when it came out. I own every expansion pack, and most stuff packs (no idea if that's helpful or not). I've had 5 star sims many, many times, in most neighborhoods, and it doesn't seem to make a difference. No one EVER asks my sim for her autograph. I now have a family of famous sims (mother and daughter 5 stars, father 4), and again.. no one asks for a single autograph, NEVER have, not once (and they are out plenty!). Anyone know why it's like this?
Don't expect them to run at you in droves. Typically (for me anyways), it is very random and not too often. Like the poster above stated, it's usually when you don't want to, as when you are trying to complete another action (same with having your picture taken).
You may try just loitering around somewhere where there are a few Sims gathered (parks, parties, etc.) and see what happens. Also make sure they are not all friends, as acquaintances and strangers are more likely to ask than friends. I think I've even had an enemy ask once.
You should consider yourself lucky, the autograph thing is soooo annoying. Everytime my sim goes out to do anything she's plagued by sims who want her autograph. A simple visit to the store become a multi hour ordeal once everyone wants her autograph or a picture.