Aha, so that's why. Schlechtwetterfront is correct. It's actually an optimization on the side of the engine. High poly models used as collision meshes are a waste of processing power, you need seperate low poly collission meshes.
I guess it easy enough to copy and decimate the original mesh. Don't forget to link the collission model to its high poly version (the high poly should be its parent). And apply the nodraw material to it.