I've sorta noticed that too, and the impression I'm getting is that they don't charge because they're trying to maintain their formation.

This is a good thing to have, obviously, because that's how pikemen and halberdiers fought - in dense blocks en masse. But the block as a whole should still probably be able to charge, so my guess is that this is a quirk in the build's AI, and it might be fixed with the first patch after release that Palamedes said would fix some AI issues.