Because someone can hold a Assault rifle with one hand
No, not many people can hold and shoot an assault rifle with one hand.
That's precisely why Blockheads aren't people.
Reguardless, if the blockhead does need two hands the weapon is obviously too long. The model looks great but is not proportioned to the Blockhead.
Because this is obviously a realistic game that needs more realistic features
I agree with your sarcasm.