That is not "all the DRM they need". Files split up into "randomly named 100mb chunks" does not make them safe from would-be pirates with a penchant for reverse engineering.
The chunks themselves would have the same DRM applied as when you'd stream from their servers. In the latter case you can also just sniff the traffic, but that's not enough to be able to decrypt the video.
DRM never did stop "would-be pirates with penchant for reverse engineering". Ever.
Plus, the owner of the Android/iOS device is paying for that content anyway, why would it be in any way a problem if he can open that downloaded file in another software?!?!
Technically paying for a license to play said content in the Netflix app.
I agree that DRM is a waste of time and the pirates are going to get the content whatever happens, but a $10/m streaming service is only licensing you to watch video in a few specific scenarios. If you had spent $3 an episode (or whatever it is on iTunes/Google Play these days) to own it, then I'd expect a DRM free file to be available (Though I doubt you actually get one in reality).