It's easy for new talents to call around to different recording studios and ask them to do their demos. Most will say yes. The problem is that most studios use the same scripts for any talent who shows up and to be honest, studio engineers are outstanding at making you sound good, but they're not voiceover coaches.
Asking a studio engineer to direct your demo session is like asking the dolly grip on a movie set to sit in and direct the film. You will not get the best direction possible during one of the most important days of your early voiceover career.