It's traditional software, it runs offline, client side. Which is great for low bandwidth places and the privacy concious. It has some basic ML for face recog. The money is in photo printing, which is a pretty big industry which Google has failed to capture any of.
It had a huge installed base, but of course Google would not sell user engagement in bulk, only in tiny pieces as ads. Advertising the things people take photos of is pretty straightforward too, another reason why Google killed Picasa instead of letting it free.
I strongly suspect that it would generally be a massive undertaking to decouple products from their internal ecosystem and render them able to be spun off.
There are several tech PE firms that specialize in that niche. KKR bought Corel (of WordPerfect fame) recently.
I'm also confident someone at Google already had this obvious idea. Any ex-googlers know why they chose not to / cannot do this?