They could, but...they don't want to. So they won't!
It would be way less convenient, though. So I don't. I don't know which device I'll end up watching on, and don't want to deal with syncing and all that shit.
Which is way more expensive than streaming services. Prohibitively so for wide swaths of the general population who don't have tech industry salaries. The cost of one TV show from iTunes can approach the price of several months of Netflix.
With copyright reform, that needn't be the case.
(Note that for many consumers, Netflix is already considered too expensive. Several people sharing the cost of one account is pretty common from what I've seen.)