>>99113075first, because it takes place in WW2, which saw the biggest tank battles in human history.
second, it was mentioned in the comics, that like real tanks, they are breakthrough weapons. They can push, they can destroy, but they cannot hold a location, and without support they fall apart.
Cruisers and Destroyers come from them being not as big as battleships.
Dunno why they used Battleships. Maybe because at the time, battleships were the largest war machines in existence.