Why do people not do it?
I don’t know the answer to your first question.
However, I didn’t want to know the gender regardless of the halacha*. I simply wanted to be surprised.
The Wolf (who, for the record, correctly guessed the gender of the first and third, but was completely wrong on the second one.)
(*Yes, I know that some people here will call me a rasha for doing what I want regardless of halacha. Too bad.)