In addition to what others wrote, here's a practical example:

I wanted to insert an old photo in a comment, from my Google Photos. The photo doesn't exist on my device, since it was taken from an old phone. I located the photo in Goolge Photos and downloaded it to my device. Then I went to pick the photo in Sync, but it was neither visible under "recent", nor in any of the albums. So I had no choice but to scroll thru hundreds of photos to manually look for it, and found it under the original date the photo was taken - which makes sense, but in the previous gallery, my recently downloaded photos would appear at the top.

I've only got a few hundreds of photos so this isn't a big issue for me, but I can imagine it'll be a dealbreaker for people with thousands of photos on their device.

It's really not, which is Google's fault. Just the ability to sidestep it for the user's gallery app of choice would be worlds better

Why not use the native picker which would allow us to use Photos for example? Seems like a waste of developer resources to reinvent the wheel.