The reason I used Google Photos was for one-click sharing. I could make a link, and viewers could pan and zoom and see all the details through a pleasant web interface. S3 doesn't do per-photo per-user access controls, and doesn't have a user interface for sharing or viewing. It's object storage, not photo sharing. As object storage, it's a lot more expensive than negatives in a binder, but about as useful.
We had to write a text editor in our first semester of our CS undergrad program. I'm not going to use my own text editor in favor of say MS word or Google doc.
Depending on contracts with an employer, there might be legal hassles as well.
It really is drop dead simple and you also don’t have to host it yourself. Every hosted provider supports external storages so you’re never locked in with them. And since you’re not actually using any of their storage you can stay in their free tier forever.
I self-host on DO and it works amazingly well.