Because it's unnecessary. The COLB he already produced and made public is sufficient to establish his status as a natural-born citizen in any court of law or government agency.
I suspect he doesn't have the long-form vault copy of his birth certificate. I don't have one of mine, and getting it is a hassle. He has better things to do than waste his time trying satisfy tinfoil-hat-wearing conspiracy kooks like you.
I suspect that even if he did produce it, scores of newbies with fake Ph.D.'s would turn up on this site and start questioning it too.
That qualifies as the most intentionally ignorant post on the subject to date. 1. The COLB posted on the internet has been proved a forgery. 2. The HI Health Dept. has confirmed that they do have his long form vault copy.
As your grandmother didn't claim to have been present at your birth in Kenya, and you aren't running for president, it's a moot comparison.