The problem is predictability of size (Score 5, Insightful)

This functionality exists in storage arrays, but there's one core problem that stop it being used in individual drives - the predictability of size. Your computer/OS is designed to believe that a drive has a fixed size (eg, 500GB). With hardware-level compression, the number becomes variable - store all video files, and you might fit 510GB. Fit all text files, and it's 2TB. But your OS doesn't understand that concept, so the drive has to present a fixed size to the OS. If it presents as 1TB, what happens when you write 511GB of video files and there's no space left on the drive to store them? Or when you write 1TB of text files and the drive is only half full.