because any crime in the virtual world can be uncovered by good police work. nobody has perfect operational security, including the government. so the solution to law enforcement is hard work by the law enforcers.
consider: prior to electronic communication, all private discussions were perfectly encrypted, because if you weren't there, you didn't hear what was happening. And society continued to function.
You simply can't trust the government to respect boundaries that they created but have the ability to breach, especially when it can be done completely surreptitiously.
We need to learn the lessons of Snowden, and fight tooth and nail to prevent anything less that complete, unfettered access to private communications by human beings. Anything that falls short of that will eventually be complete, unfettered surveillance, because there is no metastable equilibrium point in the middle.