there could be shrapnel versions of shells maybe, but I wouldnt go for anti-tank shells being all-rounders, it just doesnt sound too likely. having some sort of blast zone where tanks weren't damaged but infantry were could be interesting (assuming shrapnel could happen in a nice sphere and be equivalent to bullets)
The reason for low splash damage against infantry on shells and missiles is ... *drumroll* Balance!!! *standing ovation* If you want to be able to kill infantry, research better shells (HE, Nuke, etc.) of put MGs instead of missiles on your tank. Or better yet, bring a riffleman along to cover your ass. You know, use that teamwork thing everyone keeps talking about.