Because the bay already has a drive in it. You cannot add storage to a PS4, only replace it. I buy a TB HDD and Its a better value if I were to use it on my One or U because I am adding a TV; if I use that same disk one PS4, I have to remove the stock one, and am only gaining .5 TB, reducing my value by half!
That and isn't the PS4 Sata 3, making USB 3.0 faster? USB support also means keep adding disks, and never delete a game again.
Why would you NOT want to be able to add storage to your console, specially when both competitor consoles do just that?