It's a solved political, human-access issue that doesn't need an easily-hackable technological hammer in search of a nail: if they can get a warrant, then they demand the password or one goes to jail. Adding backdoors would inevitably give unimaginable power to a host of foreign governments, random hackers and political adversaries... little good could ever come from it that couldn't come from warrants and traditional channels of legally-compelling disclosure without the damage to the already tenuous image of the integrity, security, and trust in most every type of system. The attack surface of modern, complex systems is big enough without a hole the punches right through it if you have the master key to everyone. For example, think of the immense risk and target posed by a master escrow key database, and some random subcontractor losing or "losing" a laptop, compromising everyone.