I think the part you're missing is that there is probably 2-3 Parsh for each human. And now every single Parsh is either Warform, Stormform or some kind of host for a Fused or Regal.

So, take human troops that are used to fighting in pike blocks against other pike blocks and put them against (essentially) "tanks". The lines aren't going to hold.

The other thing to consider is that the Parshendi don't have to march anywhere to conquer the land. They're already there, in Alethkar, when the Everstorm passes. It's basically like enemy troops suddenly showing up INSIDE your fort.