Why does iOS 11.3.1 insist on reinstalling its defaults apps
This is called 'Offload Unused Apps' and was a feature introduced in iOS 11.

Source OSX Daily:


  If you frequently run out of storage space on an iPhone or iPad, you will appreciate a new feature in iOS that automatically saves storage for you. Called Offload Unused Apps, the toggle allows the iPhone or iPad to perform housekeeping and delete apps that have gone unused for a while, just as the name implies. Offloading used apps can noticeably help to reduce storage constraints on a device, since most of us have a handful of apps that may not be getting any usage but are taking up storage space on an iOS device anyway.


To change this setting:


Open Settings
Tap on iTunes &amp; App Store
Scroll down to locate Offload Unused Apps and toggle to the desired position

Exit out of Settings


If this is turned ON, iOS will automatically remove apps you don't use only when your storage is low.  To use that app again, as you've discovered, you just tap on the app and it will automatically download back to your device.

You’ll notice that while the feature will remove the app, it will maintain data and documents related to the applications that are offloaded.