It's artificially constrained in that notaries are official offices because their stamp signifies that the signature belongs to the signer. In cases where identity of the signer has to be verified for legal reasons, the alternative is to force the signature to take place in front of a judge, which would unnecessarily waste the court's time when they can have agents perform this witness for them. Further, notaries are not a profession. Anyone can be a notary by filing an application with their state court, and in many places in the US, they are forbidden from charging more than a couple bucks for their service, if they're allowed to charge anything at all. Good luck trying to turn that into a profession. I've never even met a notary who charges.