Because they have a captive market: In general the use of a notary is mandated by law, and being a notary can be extremely regulated with limited numbers of not

Because they have a captive market: In general the use of a notary is mandated by law, and being a notary can be extremely regulated with limited numbers of notaries/licenses. They could open this to competition and let lawers do the job as well, but that would go down badly with the notaries who make a fortune.