This is an interesting inquiry. There are two primary reasons;

Sometimes, no known image exists!

Secondly, it’s the preference of the “Findagrave” memorial page manager.  On Facebook, it is common to see gravestone images instead of facial photographs on birthday remembrance postings. Memorial pages on Findagrave are created by different individuals.

Although a photograph of the person is typically the default choice for the first picture, some page managers opt to display the gravestone instead.

Unfortunately, we have no authority over their decision. We can only directly control this decision, when we are the actual Findagrave page manager for that particular memorial page.  Typically, we are the manager when we create the memorial page.  A good many people monitor daily obituary postings and as new ones appear, they create Findagrave memorial pages.