"physics vehicles can't have legs"
they could if you animated them right. an at-at can be made if someone else more experienced with modeling and scripting does it.
An AT-AT being tipped over would look very silly, the animations for going left and right could be "easily" done since it has been done before on a smaller scale (see Filipe's steering wheels and the such), however going forward and backwards would have to be done by a loop in a script which is not perfect.
Walking like animations are much "easier" for a bot/playertype than a vehicle since there is more support for it.
A lot of animations on vehicles including idle animations have to be done by script making bots a much easier way to do mechs and the such.
I am pretty sure there are more reasons that i forgot or do not know.
A physics vehicle mech could work but would not be much better if better at all.
yeah doors and stuff can open on cars
it's not impossible
That is hardly comparable apart from the fact that it shows animations can be played on vehicles.
Now i am imagining a mech with doors as legs though, darn you.