It would literally take seconds to clear this up.
Which is precisely why not taking those few seconds to do so raises eyebrows. It's really no different than Obama's refusal to respond to innocuous early inquiries about his origins and qualifications.
The difference, of course, is that Obama was (and still is) protected. Can Cruz claim the same sort of protection or will a shopped Federal judge and a gleeful media derail his candidacy at some point of the DNC's choosing? That was the whole point of Trump bringing up this question. His stock-in-trade has been to point out the elephants in the room that nobody else wants to talk about.
Cruz clearly doesn't want to clear this up or he already would have, and hand waves are unlikely to suffice once the stakes (for the opposition) grow high enough.