> Is there any reason why these apps have to keep the database state locally? Why not just create a web service? The problem with this model would be keeping the server infrastructure running and getting some money to pay the bills, but it looks much simpler.

The idea of not having my personal decks stored on my own disk(s) is honestly much worse than the annoyance of sometimes (rarely) running into issues syncing. I've been working on various decks for literally years and have several thousands of cards made. Server-side stored only? Please no.