Because it's the benchmark that other photo app alternatives strive to be. It's a nightmare privacy wise, but Google built the best solution and a lot of iPhone users just assume Apple was the leader without trying anything else.

Thankfully there is immich which is working hard to duplicate the feature set without the privacy invasion.

Apart from the fact that they probably use your photos for training, it's pretty good. Searching by people, locations, text or objects in the photo is awesome (I know Apple recently added something like that, but Google have had it for years).

Also the storage allocation is shared with Google Drive, so if you already use/pay for it, it's included.

I use it to back up my photos and create shared albums during vacations.

Because it's the best option I've seen and it's one they haven't ruined?

Cheaper than iCloud, superior tools, less issues with connectivity.