Security measures - Sony doesn't want you doing things like that because it gives people more opportunities to find exploits that will eventually lead to piracy (which will reduce its revenue).
Are these security measures not bypassable? This is the same thing that Apple tries to avoid in iPhones, but AFAIK is possible to change the firmware for jailbreaks.
People have tried. The whole reason they removed the OtherOS functionality is to get around the one known public break in their trusted computing model.
