See basically a private key is an integer between 1 and 2^256 converted to hex right. So what's stopping Random number generator to pick a private key from say first few million integers??
What's stopping the random number generator from doing that, is
chance. What you're suggesting has a chance of 1 in (give or take) 100000000000000000000000000000000000000000000000000000000000000000000000 to happen. It's not going to happen, don't worry about it.