As notaries are appointed by the Govenment, their fee is regulated.
The office of the notary is a private enterprise, which is only paid by clients. But notaries must (obligation of the govenment) avoid competing against each other. That's why the price is fixed by the Govenment. That's also the reason why the access to the prefession of notary is limited by a 'numerus clausus'. In several ways the Govenment limits the number of lawyers who can become a notary. Notaries aren't alowed to make advertising. It's all part of the idea that they shouldn't compete against each other. In Belgium there are several businesses like newspaper distributors, pharmacists, and even taxi companies in the city of Antwerp, were the access to the business is limited by number by the govenment.