Presumably there is some level of traceability to this. If a falsified photo is found on a document used to commit fraud or other crimes, the photographer who enabled it would be banned, blacklisted, and possibly held criminally liable.
Unlike the old "guarantor" system where a licensed professional or public official (judge, lawyer, doctor, etc.) had to sign your application - basically, people who the government could verify the existence of and hold accountable - there's not very much traceability to the photo system.
As an American immigrant to Canada, I don't see the point - the US has never in my lifetime had either a professional photo requirement or a guarantor requirement for its passports, nor even a digital requirement.
Certainly, paying someone like a pharmacy for passport photos is not rare in the US to reduce the risk of out-of-spec photos leading to a rejection. But that's just a common personal choice and not obligatory.
I suspect Canada's guarantor system is inspired by the UK's, though I don't actually know.