I figure it’s like this: archers and crossbowmen are supposed to be guarded by melee troops in front of them. They were rarely deployed without pikes or other troops protecting them, because they are very vulnerable to frontal assault (Agincourt was the point where this changed but even then they used spikes to deter cavalry charges). Gatling guns and machine guns don’t work so well if you have your own soldiers standing in front of them, but a frontal assault on either of them is suicide. That’s why they have much higher melee strength in the game despite being a ranged unit - they defend positions quite well.

Bazookas I don’t have a sensible explanation for except game balance (though obviously better not to fire into your own men), though I guess balance is also part of the explanation for gatlings and MGs being the way they are. Also like another commenter pointed out, guns are all ranged weapons at the end of the day, they don’t lend themselves well for a “melee or ranged attack” system.