You can buy an iPad Pro with as much as 2 TB, which obviously isn't nearly enough to edit feature films but it is plenty for people doing say 30 second ad spots, music videos, or other short subjects.
This isn't intended to replace all uses of a Mac/PC, just some of them. Even if you are working on a longer project that is way bigger than 2 TB, if you loaded in a short segment you could work on that while you're on a train or sitting in the airport or somewhere else that you maybe don't want to drag out the 16" laptop that contains the entire thing.
You might as well say "why are these products supported on a laptop, if you are doing <insert whatever you consider 'real work' here> you need a beefy desktop machine with an attached storage array!" Apple is not claiming this will replace all uses of a Macbook Pro or Mac Pro with a tablet, so stop whining that it can't.