Upvote:9
There are several reasons why they ask: the most relevant is that when the form was designed, they either didn't have that information any other way, or didn't have it an easy way the officer could see while talking to you.
Of course, the form contains all kinds of information they could look up. For example, it asks for your flight information, even though you've provided your name so it could be looked up. Today's date, even though it's, you know, today.
The forms are becoming obsolete, so redesigning them to ask for less and less information is not going to happen.