It’s because Android is designed to run every goddamned app you have installed just on the off chance it needs to phone home and/or gets a notification. You can’t keep apps installed that you only use once every few months or something, or they’ll be run in the background without notice, too. Android gives the user little to no control over this.

Every app gets run, then swapped out to cache files (where all your space is going) because it makes Android seem “faster” (especially on older or more limited hardware) as the initialization routines of the app are already done - all it needs to do is bring the cache files back into active RAM and it’s ready to use. Nice if you actually USE all those apps constantly, but if you have a large number of apps installed while only using a small subset of them regularly then it just results in wasted storage space, CPU cycles, battery power, etc

There are a number of apps that will “freeze” most other apps to try to prevent them from running, but it doesn’t always work (I’m not an expert, so I’m not going to try to explain why as I’ll probably get some details wrong). Nonetheless, they are helpful enough with most to be worth using, IMHO. I personally use “Hail” with “Shizuku” (for the elevated privileges necessary). Both are available on F-Droid.

ETA: Another option is to “back up” the APK file (there are a number of apps that’ll do this) to local storage, then uninstall the app & only install it when you need to use it. Recent versions of Android will let you keep the app data so it’s there when you need it upon reinstalling. Just make sure to clear the cache to free up space before uninstalling (might work afterwards with a cache cleaner, too - not sure).