Very simple… They are bigger then the player models so when in close proximity to them even if not aiming at them they cancel out your bullets. Makes shooting down a corridor or sniping (the issue that I have) really difficult when close to friendly players.
Please understand I am not talking about when shooting at a player… I think that is fair enough.
Sniper rounds pass directly through players (including friendlies) and still deal full damage, team-mates strafing in front of you is part of the reason for it but also because it allows multi-kills 
so I could line up 5 friendlies and 5 non-friendlies shoot though them in a line and kill all 5 enemies? thats cool.
Well I know that I have been having issues with normal weapons as well and just got frustrated due to trying to shoot someone in the head (not moving) twice. then I take 2 feet to the side (away from team mate) and boom headshot. I will look at the area again maybe its just invisible walls again -.-
We could also probably use per-pixel collision or smaller hitboxes for friendly fire. I’ll ask the clever programmery types.
I am not a programmery type however I believe that would add a HUGE overhead due to both the complexity of the models and the fact you would be either doing double the number of traces OR doing more checks per current trace. From a performance standpoint it would make more sense to allow bullets to go though friendlies BUT still do damage if it hits their hitbox… its kinda the best compromise.