It's because you're a lot more afraid of being KOed yourself. I can't give you a comparrison to Thai boxing, but if you take organized bare knuckle fights etc. the fighters generally keep their head pulled back which takes SO much out of their punches. Also, the only time a knockout really occurs is when someone is running forward and goes into a punch because their reckless attack evens up the power level.
It's like a more extreme version of the difference between MMA and Boxing striking. In boxing, SO many more punches land because the punches don't hurt as much due to the padding. They are more willing to take shots because the fear of being KOed isn't quite as strong. In MMA, the gloves are a lot smaller so there isn't quite as many punches thrown due to the fear of taking one back and how much more damaging that shot is. Cross that with bare knuckle, where you also get the fear of breaking a knuckle or a hand, it usually means for a lot less proper punches.
Thanks to Ape for the sig.