Why do Android phones update so much?

Android app ask for updates many times in a month because they work daily on their app and improve and fix the problems. When new bugs and features suits they implement on the app and put them as a new update. Or they recommend through their app for update.

Why is my phone constantly updating?

It’s normal for a phone which is running an earlier version of the OS when you buy it to update through several versions of it until the latest available one for it has been downloaded and installed, if that’s what you mean.