Everything that can happen, will happen. And finding a private key with a balance can happen, however small the chances are. Therefore it will happen. The odds are small but that doesn't make it impossible, I don't think that people seem to understand that just because the odds are greatly against the possibility of it happening it doesn't mean that someone on their first try on keys.lol can type in a random number and find a wallet with a huge balance attached to it. I personally have created a webscraper running that randomly check private keys so if I never find anything major it's cost me basically nothing but the time it took to create the scraper which was only a few minutes. Not even electricity as my pc runs 24/7 anyway
Is it possible? Yes
Is it probable? No
You clearly don't understand :
1. How big is 2^256 possible private key and 2^160 possible bitcoin address. Please calculate how long it'd took to get all possibility with today's supercomputer.
2. Most wallet uses CSPRNG which won't let people guess your private key easily.
Simply brute-force all possible permutation won't get you anywhere, you must reduce space search either by finding flaws on CSPRNG used by a wallet or knowing whether a user use insecure way to generate his bitcoin address/seed.