Well as JZ pointed out, in Epic the term "bunker" represents a number of different structures and defensive positions. Having actually constructed two man fighting positions to being in a number of different designed types of bunkers. As well as being run over by an M60 MBT while in concrete reinforced "foxhole" for training purposes.
IMO, with all the options of what Epic considers "Bunkers" ... probably for simplicity ... making them Impassable was a good compromise. As WEs are generally very heavy. And if the "bunker" they were on collapsed. The WE could be damage of get stuck. Even barbed wire & concertina could snag a tracked or wheeled vehicle's in the various suspension system designs. And could do enough damage to make them immobile.
For a say ... a real world example. And I've been told about this incident a few times. At an SF base camp somewhere in Vietnam. During an NVA attack, a Russian made PT- 76 Light Amph Tank. Parked on top of a well built and designed bunker. Most of it being underground.
And of course the SF and others in the bunker were calling back to Higher HQ to inform them of what was going, etc., a SITREP. Not to go into the conversation too much. But the Intel Officer in the HQ, said, "There is not any enemy armor in the area, based on intel estimates, etc. The SF NCO replied, "Well if you wait a minute. I'll let you talk to the PT 76's driver".
The point being, IMO, had that not been a PT-76, but a larger heavier AFV, like a T-54, the bunker might have collapsed. Not only killing or wounding the occupants inside. But there would be a good chance the T-54 would have become immobile/stuck/ throw a track, etc. ...
So I think the Impassible rating for WEs may have some validity. Imagine you are walking along and a hole is covered by deadfall. Or purposely put there as booby trap, etc., ... So you don't see it. And step on the hole. You could sprain an ankle, etc., ... Been there ... done that ...
