Because history teaches us two lessons.
First, it teaches us that history repeats itself. Second, it teaches us that most people don't learn from history.
Remember the NYC gun registration, back in the 1960s? All guns had to be registered. All guns. And the lesson is apart from the nonsense entailed in the registration sceme (it wasn't "just" registration -- you had to file lots of paperwork, personal references, and then, get permission from the NYCPD -- after paying a nice fee).
When they first proposed that law, you see, the "spiritual forefathers" of the nastymouths in this thread mocked and taunted anyone who even suggested that they'd use the registration records to confiscate any of the firearms.
They were told to stop being paranoid, it'll never happen, don't be silly, show me where it's been abused, blah blah blah.
But then, guess what?
Ah, you guessed!
Yup. They used the registration records to confiscate firearms.
Any anticonstitutional power will be abused. It's not a question of "if", it's only a question of "when".
To argue that it hasn't been abused yet is beyond absurd. It's surreal. It's like finding an intruder in your house, and reaching for the phone to report a burglar, and then, the intruder protests: "Have I stolen anything yet?"