Limp Calling is so weak.

99 preflop is such a tricky hand to raise preflop with because if you hit a set on a weak looking board, it's nearly impossible for anyone to put you on your hand. In these situations a c-bet will often look far too suspicious to your opponents, sometimes so much so that they re-raise with air, allowing you to slow down and let them bluff at the pot, setting up check-raises etc.

And the times overcards hit the table? A c-bet will often take down the pot.

Isolate and punish your opponents, raising pre flop with 99 is a strong and +EV play in most situations.