Because iCloud is a big part of their lock-in strategy. The issue is not about transferring a few photos adhoc from a phone. It is about having potentially terabytes of family movies and photos stored and organised on iCloud. Like I said, there is a financial incentive for them to make it difficult for people to move away. There is no easy way to transfer everything off, including all metadata.
It's pretty well supported?
Ive personally used the export / archive features of iCloud when i went from iOS to android. the longest part was waiting for the pics / vids to download from icloud and upload to google drive.
I even exported stuff out of google's ecosystem and back into iCloud when I moved back. This was a month or two ago.
I know many many years ago I had issues with iCloud, but that was before they fixed / improved their web interface (and everything else).
I was talking about the OSX "photos app" - an iPhone typically won't have enough storage to hold all photos at once, so doesn't seem like the best way to transfer photos away from iCloud.
What does the iPhone not having space to store all of your photos have to do with using iCloud?