It's a bit ironic, considered that WC3 introduced atmospheric combat...
Previous designs look like they could fly in atmo, WC3 ones don't (except the Hellcat, but you don't fly atmospheric missions in it).
WC4 is better about it, Bearcat and Dragon look like they could fly, Vindicator and Avenger look like they could, but barely, only Banshee doesn't look atmospheric at all.
WCP has more aerodynamic designs (except Wasp and Devastator, most of it's fighters would fly in atmo), but it lacked atmospheric missions.
It may have to do with available technology, boxy designs and non aerodynamic ships take less polygons to make, which was really important at that time (remember, we're talking about the very beggining of 3D technology here).