That assistant's job is to set a schedule.  Almost all provider's will prescreen for convenience and then if it works out, it works out.  But there are several providers who it wouldn't likely work for:  Those with childcare issues (hard to say I need a sitter sometime for a booty call and have them available).  Others have family time or private time they don't want interrupted, maybe a relationship outside of the job.  And there are huge numbers of us that simply like to know what we are doing and when so we are prepared physically and emotionally for the meeting.  Not everyone's life is built for dropping whatever they have going to suit someone's late night needs.