Because, if it is true, there is no reason not to produce it.
Let us not forget his college records, either.
Rush agrees and went out of his way to make that point, too, further down in the transcript:
RUSH: I checked the e-mail during the break and quite naturally, a lot of people say, “What do you mean, Rush, you haven't made a big deal out of Obama’s birth certificate?” I haven't. You ought to see the e-mails I get from people who call themselves “birthers” who are livid at me for not picking this up so what I did during the break I went back to my own website, two things I found. First, July 31st, 2008. This is from the Stack of Stuff Quick Hits page, and I'm just going to read what I said: “Oh, that birther thing, you know, the birth certificate thing. Andy McCarthy wrote a brilliant piece on this whole birth certificate controversy at National Review Online. Really, he says, it's not about the birth certificate. It's about Obama’s honesty.
“He hasn't released his law school records, his law review records, his Columbia records, his university records. He hasn't released a lot of things. This thing could easily be shut down if Obama would just release the birth certificate. You remember the Drive-By Media and the Dan Rather business? Dan Rather, faked documents. Mary Mapes and Dan Rather faked documents to show that Bush skipped out of National Guard service, and they ended up giving Dan Rather an award! They circled the wagons. Peter Jennings and Brokaw, they gave him an award for great work. They circled the wagons. They made it up, and now, ‘This is just a bunch of kooks. The birthers are just a bunch of kooks,’” about this birth certificate.
Oh, yes there is.
Those who vote for him or could potentially vote for him believe he was born in the U.S. or could give a rats ass and see "birthers" as a bunch of loonies.
As long as his opponents look and act like loonies, he wins.
If he were to produce evidence which would satisfy "birthers" then he looses a wacko opponent group. That would not be smart politics.
So the status quo is the best for both sides. Keep doubt about his legitimacy on our side (which is good for the base) and the left gets to see what it wants to see - which is good for their base.