Person buys a phone, it's not got massive amounts of storage because person is not rich. On the box it says the phone has 64GB of storage (we could call that RAM, but it's not like a computers RAM as it's persistent). Person turns on the phone, and it says 'Hi, I need to update all the provided apps.", The thing is, the provided apps, are in ROM, and do not impact storage (other than for their data), but the updated version from the App/Play Store is now stored on the main storage, reducing that 64GB the phone came with before you've even had a chance to use the phone. ROM is also faster than the storage, not by much but it all adds up. Apps in ROM are quicker than in storage. When apps are 'sleeping' their run states are swapped out to storage, if storage is low, then the phone constantly has to make decisions on what old run states or temporary data needs to be purged first to make space for the apps run state to be saved. If a sleeping apps run state is purged, then when you click on it's icon again, it has to start up afresh, which is slower. All these things add up to delays. Then there's the constant indexing of your photos, videos and messages, the more stuff to index, the bigger the database, the more time and CPU cycles it takes to maintain it.
Basically, it's phone death by 1000 cuts.
What I do on any new phone or factory reset is NOT update any app I never intend to use, and/or disable it if I can. I also regularly purge older photos and videos (after having backed them up to another medium first).
However, none of this will stop newer apps just being more complex, and eventually your phone can no longer keep up. Phone app developers rarely optimise their apps for slower hardware, as the 2-year phone upgrade behaviour is now so strong in most users.