Citations are too open ended and prone to variation, and legitimate minor mistskes that wouldn't bother a human verifier but would break automated tools to easily verify in their current form. DOI was supposed to solve some of the literal mechanical variation of the existence of a source, but journal paywalls and limited adoption mean that is not a universal solution. Plus DOI still doesn't easily verify the factual accuracy of a citation, like "does the source say what the citation says it does," which is the most important part.

In my experience you will see considerable variation in citation formats, even in journals that strictly define it and require using BibTex. And lots of journals leave their citation format rules very vague. Its a problem that runs deep.