A friend once pointed out that it's likely a majority of "amateur" porn is likely private content from hacked or stolen accounts and wasn't posted by the any of the parties depicted.

He mentioned this when a bunch of stories were coming out about GeekSquad and other IT help as a service companies stealing data or acting as data harvesters for the FBI/DEA etc.

I don't really understand why people even make their own porn, but that aside, I really don't understand why they would save it in the cloud.

Most likely the thrill of it. They might not even be aware of saving it to the cloud. Maybe they used their phone on a stand to record and iCloud or OneDrive or Google Photos just synced it automatically.

If I take a photo or a video on my iPhone, it's uploaded to iCloud automatically, and afaik there is no way to remove it from iCloud while still keeping it in the photo library on the device without opting out of iCloud Photos entirely.

Defaults are powerful.

I don't think that's likely at all. It seems like it would be far easier to find women who are willing to take their clothes off for money (something that has been relatively easy to find for centuries) than it would be to hack hundreds of devices in order to steal such pictures - if they happen to exist.

This should really be treated as theft on a massive scale. It's similar to if he broke in to 300 peoples houses. Probably even worse.