Modern bombers no longer need to be tanks. Modern warfare prioritizes getting to a battle and having a bunch of bombs more than surviving weapons the enemy does not have.
Bomber planes in 1944; Bomber planes today; I can survive an onslaught of German fighters, heavy flak, and two engines being taken out and still limp back home; if a bird hits my engine i am grounded, but at least i can fly faster and carry a heavier payload