Why are Celebrity Producers/bookers rarely considered for Casting Producer / AP roles?

I see many jobs on here for 'Casting APs' and 'Casting Producers' - asking for experience booking talent / casting actors and dealing with agents.
I feel I am qualified for some of these positions but I'm not allowed to apply for them, despite the job being similar to celeb booking. Or at least I think it is. I've cast many celebs to host TV shows, appear in ads, on panel shows, & interviews. This requires skill in casting, so why can't I apply?