You don't "have" to. And you may appreciate those land units if your opponent ever casts say ... Storm? Or has a Staff of Storms that applies before combat movement, or ... Perpetual Storm?

Against Independant units Mammoths and Wingless are very nice.

For the sacrifice of being able to fly, having high movement them being weaker in combat than a normal unit is perfectly acceptable if you don't take the standard "I'm stronger I kill youz" approach.

Maybe they take more finesse than you are used to and that is why you do not appreciate all the facets of it, I do not know.

If you want a totally flying army that can stand up to non-flying counterparts maybe you should submit a 'balance' or 'theme' request and not ask how to play what was placed before you.