[TopTheThaiGuy]

Speaking from my limited experience developing my own app, 3rd party log-ins are a mess and not all use the same protocol. Most use OAuth but some are proprietary(ish). And to have user signing up in-app, you have so much more control, in terms of informations you need prior to giving the user access. In my app, If a user sign-up with 3rd part service, and have to block the user from getting into the app before I have more info from them(necessary data). I think it's pretty janky.