The simple fact that training for competition is not the same as training for battle. Street fights and war have no rounds, no time outs, no DQ, no ref and rarely any rules.
The NHB fighter may have better cardio, better pain resistance and tolerance, and more strength. He needs it, because, like a boxer, he must withstand a number of punishing blows while getting into postition to try and twist or choke someone into submission. He isn't worrying about the guy pulling a blade, or someone hitting him from behind with a bottle. Its 1-on-1, unarmed.
Next time you roll, do it 2 on 1. Add in a pair of rubber (ie safe) training blades. It changes things. Considering that almost 100% of competitions are unarmed, and over 60% of street confrontations are armed, that makes a difference. All the cardio and all the endurance in the world isn't going to save you if it's a 3 on 1, and they're packing heat. The only thing you'll be packing is your shorts. Mike Tyson's a bad ***, but he isn't worrying about Hollyfield pulling a glock in the 3rd round. So I doubt he trains for that situation.
Show me a NHB/UFC training course that includes weapon use and disarms, street smart tactics, etc, and I might change my attitude.