This sounds very convenient until one fine day you decide add a new feature, so you need to change your code, but the new code has different variables and there may not be any sensible way to "port" the data.
So in this sense it could be a bit like creating migrations to your database. Maybe on ”cloud scale” you just would like to migrate the data when it is used.
Edit: Few more details in this document: https://sdk.dfinity.org/docs/language-guide/actors-async.htm...
Because that's not probably what it happens...
It's an idea that has been tried on and off for decades and is obviously very appealing but tends to have lots of interesting problems when it has been used for a while - often when you want to access your data outside the context of your application.