People can guess private keys? Seriously? No one is capable of doing that genius. Many have tried to crack these keys in different ways(Brute force etc), but failed which is why a large portion of BTC is lost basically.
Supercomputers could possibly crack them in the future, but it would still be really difficult.
Of course, it is not now. It is a potential problem. When the number of Bitcoin accounts is large enough. It will be easier and easier.