In the US and many other countries (I can confirm this is true for Japan), pharmacies are also convenience stores, and sell many other things besides drugs.
I did once hear that part of it is historical. That in some places, many years ago, because photo developing was a much more manual process using more primitive means, and given that pharmacists of the time were used to handling and storing chemicals, and carefully following chemical processes and so on, that there was a natural link between the two. This would be back before the time of posting your film to Snappy Snaps, or there being a centralised photo-developing laboratory with an associated chain of collection huts that simply acted as the interface to the public. Once an association is created - exposed film goes to the pharmacy to be developed - it tends to linger.
However, I've not been able to find any evidence of this. It could just be a just-so story.
https://www.cvs.com/
For instance, here's a US pharmacy chain – notice it mentions photos on the home page.