The thing about stowage is that it needs to be thought through to look realistic and most gamers don't, they just pile it on wherever.
Tanks move. They move as often as not over uneven ground. Items simply placed on top of hull or turret won't stay there long unless tied, wedged or otherwise secured. There's a reason stowage bins exist. Few enough bother to model the cords, ropes, straps, chains or bungees or place the stowed item against a tie point/ grab rail. Most gamers stowage seems to be impelled to adhere through magnetism.
Tanks can get quite hot, especially around exhausts and cooling ducts. Nobody with any sense is going to wedge their bedroll against a hot pipe, that's likely to set it alight. They are probably not going to do the same with cardboard ration boxes, wooden crates or metal ammunition boxes, if they have ammo in them.
Turreted vehicles, where possible, like to take advantage of the possibilities inherent in rotating that turret. Untold exceptions of course but given the option people will try and avoid blocking the rotation of said item and certainly discourage the stowage of items where they will jam the turret.