In fact, I have already found a few private keys (unfortunately they were already emptied before by someone else).
If I choose a number between 2 and 4 and not choose 2 and 4 themselves and you guess my number that doesn't mean you can read my mind.
That is the simple way of saying what you have found is not a "normal" private key. You have found the solution to some sort of puzzle where you searched in an extremely reduced search space.
~ run such brute force attacks for random keys ~
Why do you think those keys were "random"?
I'm sure that North Korea and other big malicious actors would be running far bigger operations to brute force random keys.
Not just NK but if all the 194 countries joined forces and tried brute forcing bitcoin private keys and continued doing it for decades, they still wouldn't be able to find any.