Imagine you have a 3 digit PIN code. That’s 1,000 options to brute force. If you remove 000-009 as “too weak”, you still have 99% of the search space available. These numbers scale favorably as the private key size gets bigger.

In short, raising the floor doesn’t really hurt the search space meaningfully.

It also acts as a test case. "Hey seed-generator, give me a random number between 0 and a gazillion." "Does 4 sound good?" "No, it sounds like you're obviously broken."

In theory, all values would be equally random. In practice, some results are exceedingly more likely to occur when something's wrong with the algorithm.

While any given outcome of flipping a coin randomly N times is equally likely, if that series looks like [heads, heads, heads, heads, ...], then there's probably something other than pure random affecting the output.