Wallets can be cracked of course, it's just that it takes computing time.
I think the average PC can crack a wallet in a few years or at least
current supercomputers can. Just takes enough trial+error
(brute force) computations.
If you have a copy of the wallet.dat file and are talking about cracking a weak password, I suppose it could be cracked through brute force.
On the other hand, if you are talking about brute force cracking a private key from a bitcoin address, it isn't going to happen unless some mathematicians discover significant weaknesses in all three of the following algorithms:
- ECDSA with the secp256k1 curve
- SHA-256
- RIPEMD-160
Without some significant discoveries of serious weaknesses in all three algorithms, there isn't enough time or energy available to brute force the private keys regardless of how powerful the computer is.