They have to do something while they’re sending all your data top the mother ship.

Actually, in a lot of cases it’s probably because the app is just a wrapper around a web UI, via some bloated system like PhoneGap. That’s why your compass app is 128MB.

There are UI guidelines to make apps show something however useless it might be. https://developer.apple.com/design/human-interface-guidelines/loading

I guess most developers go for a logo rather than a spinner. Maybe they worry that folk will forget what app they tapped on?