"Photos" not in "Open With" drop down list. Why?

No. The Photos app doesn't have an "Open" function for photos or videos like most other applications have for their document types. You have to import photos or videos into a Photos library before Photos can do anything else with them.

You'd think they'd have a way to get Photos into that list.

Only if you misunderstand what Photos is. It's not a pixel editor. It doesn't actually edit photographs at all. It's a media manager with built-in non-destructive processing. It's database driven. In other words it's a parametric editor. It never changes the original image at all, but rather preserves it the way a film photographer manages his negatives. Your editing decisions are recorded in its database and you can only access those edits by exporting from the app. This contrasts with pixel editors that do actually alter the file when you edit. Apps like Photoshop, Graphic Converter, Affinity and so on.

So as you don't want to use Photos to manage these apps on your desktop, and presumably don't want to preserve the original, then you need a pixel editor not a parametric one.

Feel free to send Apple feedback:

Product Feedback - Apple

But the problem is that if Photos is on that list and automagically imports the file, it does a lot of almost irrevocable changes. If you "open with" the file in Photos and decide "oops, wrong picture", it can take a lot of work to back it out.