Because it is only technically not airborne. Airborne transmission requires it to be transmitted by a droplet size that can stay in the air for long periods.

Ebola can be spread by droplets between .8 and 1.2 um. A sneeze creates droplets between .5 and 5 um.

I'm guessing that sneeze size droplets can't stay airborne for a long time.

I'll stick to the safer course and treat it like it's semi-airborne.