That's bad logic. You're making the bombers ever more powerful in defense, in what they can destroy in 1 pass, in how they can affect OTHER players' gameplay, at an unhistorical benefit to the bombers.
You also might as well complain that 4 bombers are "soft" compared to 6, and 6 are "easy pickings" compared to 12.... There's no reason, by your logic, not to have 1 player invulnerable inside a 100-bomber formation.
You have to think about the impact upon gameplay this would have. It would be detrimental.
P.S. 2 bombers are still quite able to defend themselves in most cases. And if they cannot (if, say, they aren't B-17s or B-24s), that's why you have an escort.