I'm trying to understand why cryptocurrency private keys are safe. Maybe it's a n00b question, but here I go.

I understand that if you know someone's private key, this is means you can transfer the coins. I tried this with my CGA wallet, I used dumpprivkey to get my private key, deleted my wallet file, restarted the client, and all I needed to to to get my balance back was entering "importprivkey ". You don't need anything else than the private key.

What if someone ran a powerful computer which runs a program that does nothing else than:

1) generate random key
2) importprivkey
3) return to 1

Wouldn't that mean that this program could randomly pick up users balances, like some lottery?