Question about Projectile Motion? Physics?

I'm not looking for someone to solve a problem. I have an exam soon and there's one thing I just can't seem to understand. When dealing with projectile motion in 2 dimensional kinematics, I understand that the vertical component of the acceleration is -g or -9.8 m/s^2 , but why doesn't the horizontal component have any acceleration? I'd really appreciate if anyone could clear this up for me! Thanks!