Android phones are multi-user, so if they can do it then Apple should be able to.
That is about one fiftieth of the work that needs to go into the feature the OP casually “why can’t they just”-ed.
This is called whataboutism. This particular feature aside, sometimes there are very good reasons not to throw the kitchen sink of features at users.
And how do you explain your 1TB phone that has 2GB of data, but only 700GB free?