It may be minimal, but why can't a HE round deliver splash damage against vehicles aswell as Infantry?
That's a crappy answer. Doesn't have to be devastating, just a little bit would be consistent. Splash damage doesn't do much against infantry in the first 2-3 shots, just add the same against vehicles.
I used to run an HE lightning with radar while with armor groups specifically because it did so little damage and C4 was so common. I could shoot right beside sunderers and do minimal damage to them while eliminating the threat. It wouldn't even do that now.
The only vehicles that would make sense to take splash is the flash and the harasser, both are already super squishy no need to make them die to rounds landing near them too.
It does... against Flashes! Flashes only resist 25% of all splash damage. Against all other vehicles though... they resist 100% of all splash damage from anything. Bulldog, rocket launcher, Fury, anything. This includes HE tank shells. Although I'm not against some HE damage in principle, but the PS2 armor damage model is just way too simplified for it to work in a balanced way. In World of Tanks, HE shells work because sometimes you simply can't penetrate with an AP round... and the meager damage of HE is better than an AP round that bounces off and does nothing. HE is also useful there for destroying vulnerable parts of a tank even if it's fairly useless against the tank itself. But again, PS2 has neither realistic armor nor modular tank damage zones, so there's none of that involved.
Because shrapnel is far easier to stop than tungsten darts and shaped charges. Realistically, HE would barely scortch the paint of a tank, but since SOE decided that being able to switch ammo on the fly like actual tanks can would be too hard to do, that would render HE more worthless than it already is. I do think splash should hurt Harrassers though.