I would refer you to the below picture to help you understand the probability of finding a "random" private key with coin in it:

There are 2^160 possible private keys. For comparison, there are currently approximately 2^26 unspent outputs, however some addresses have multiple unspent outputs.
If a private key was generated
randomly, for all intents and purposes, it is not going to be found out via a
brute force attack.