“What is wrong and embarrassing is the President of the United States reciting a massively discredited factoid.”
Because the useful idiots of his coalition believe it. And that’s all the reason he needs to lie – the lie is useful.
Not to mention than none of the Democrat hacks with bylines will call him on it. The other reason is that in order to understand why the claim is bogus, you have to understand a bit of math and some statistics. That lets out two-thirds of the American people, and seven-eighths of the lefties right from the start.