This is a very complex question.

Part of the answer is that they think the surveillance will be magically omniscient, because it's technology they don't understand.

Part of the answer is that they think that if there's a tool they could possibly have to give law enforcement more power, they must have it.

Part of it is that they don't care so much about actual bad guys, but about exercising absolute control over the general populace.

Part of it is that they don't believe that crime can actually be eliminated, but they do believe that they have to continue to take all possible measures against it.

And part of it is just that they don't think it's politically safe for them to oppose a measure like this (similar to, but not quite the same as, the second point above).