[Mobhack]

Bunkers are not steel - they are concrete, logs or earthworks considered permeable to fire in the SP universe. Even rifle fire will cause occasional casualties to a maginot line type mega-bunker.

They are really just field works in this game rather than say the sort of WW1 type of deep bunker that could resist a Somme type barrage over weeks. A "SP WW1" Somme bombardment would kill everything in such a fortified line in a game hour or 2.

That is probably done for game play balance - you would have to ask the original SSI team as to why. The same applies to the ease of clearing tactical obstacles like wire and mines, DT etc.