It’s more like the underlying system works. Since there are no “half cells” (at least currently), it can’t really prevent stuff from moving in diagonally.
The game would need to know, if something is already in the vehicle, what defines as vehicle and what part of that vehicle is closed. Then it would have to decide which part of the vehicle takes the damage when it’s attacked. It also should not alter the collision boundaries, otherwise you’d unexpectedly hit stuff where you think your car would fit just fine.
There’s a bug report about it on the game’s GitHub since early 2014, even with a 15 $ bounty on it for anyone who can fix it.
You find a zombie-proof(ish) vehicle design over in another topic, which also includes a link to the bug report mentioned:
Searching through the forum with keywords like “diagonal vehicle” will also yield more on this, including discussions about possible solutions.