A large part of it for me is just how easily destroyed defenses are and how long they take to set up, especially compared to the first game. A ratio of effort expended to fortify vs. destroy, if you will. Sandbags get one-shotted by tanks a lot of the time. Tank traps are only available for USF without doctrines, otherwise I'd tend to use them more.
The second big reason, as I already alluded to, is just that every faction doesn't have all the available defensive options. Ostheer have no sandbags or tank traps, Soviets have no tank traps (at least not in a good doctrine), OKW have sandbags but no tank traps, etc.
But your post does make me wonder. I may have to utilize defensive structures more. I don't tend to get creative with funneling units into kill zones. I think it would be a really interesting way to play. I'll have to experiment with this.
I remember seeing one of Dane's videos that highlighted a 2v2 where the one soviet player went defensive tactics and made great use of tank traps and barb wire. He essentially made a funnel and directed enemy units straight into his mgs, anti-tank guns etc. I think this was a vid from a month or two ago. Great game to watch. I think proper defensive networks just take too much time, and pull your attention away from the battle too much.
I'd love to see that. I bet it would be interesting; could you provide a link?