OH, in WW2, i.t was standard for all armies. Because APCs really came late and in relatively small numbers, and IFVs were still like a decade in the future. So how are you going to carry your infantry around, if you're not fully motorized and later mechanized like the UK? Well, have them ride on a tank. Not like they're gonna keep up with the panzers on foot or with horse drawn carts for Germany, amirite?
(Though in the USSR, the cavalry obsessed Kliment Voroshilov did come up with, hey, let's make divisions with one tank regiment and 2 cavalry regiments. Because nothing ever went wrong with sending cavalry against stuff with machineguns. Seriously, the guy actually had Mikhail Tukhachevsky, the proponent of armour and motorized/mechanized warfare purged, for not being one of the good old cavalry guys. But I digress.)
But you know, even back then it was obvious that it's an advantage to have some other mean of carrying the rest of the tank division than them riding on the tank. Not just because of protection, but when the composition of a division settled to about one platoon per tank, plus the other components like artillery, engineers, etc... you know... where the EFF are you gonna fit all those on a tank?
Yeah, that's the composition problem even ol' KV faced, as early as 1941. You literally can't fit 20 to 30 guys on a tank, so maybe use horses?
At any rate, WW2 is WW2, and BMP times are quite a bit in the future. The whole POINT of the BMP was to be an APC that carries a squad close to the battle inside, dismounts them, and then fights alongside them. The whole POINT of the BMP was to replace the trucks of the motorized infantry part of a tank division with something that was both more survivable AND more useful in the actual combat after dismounting them. You know, by being able to also fire machineguns and lob bigger projectiles to support them.
The current Russian idea of carrying them on top and then buggering off to leave them to fight alone is really neither. It's literally a worse use of an IFV than just using a GAZ or Ural truck instead. I mean in a truck they're at least partially behind the engine block, and it doesn't have internal ammo to cookoff
