Depending on the type of pro (soloist), they might have their music memorized. If it's an accompanist, I would think the page turner is like providing the piano. House provides it. And then there might be the few exceptions who travel with their own piano and maybe their own page turner.
Actually, realistically I could see someone being a professional page turner. If it's a husband and wife "team" where one is the performer and the other is more of a manager, I could see the spouse also turning pages. It wouldn't be their sole job responsibility but it would be part. From the performer's side, there's peace of mind knowing the page turner is competent, can read music, maybe even knows the piece, etc. Or if not a spouse, then a manager or someone who is traveling around with the performer.