Because guys are intimidated by beatiful looking women. Why?  They  are scared of getting turned down. Because of their low self-confidence they automatically conclude that they aren't good enough for them. Making them feel bad, unworthy and unlikely to engage in conversation, or approach them. 

This mentality is so stupid that its laughable 

I think like that myself and Ican't help it so I know where you and the rest of the guys feeling this way are comming from. But let me tell you that its the worst thing one can do to himself. While out partying I was approached by woman in her 40's. She was a bomb. A supermodel. And she wanted me! 

After that day I realized that there is nothing to be shy off or to feel intimidated by beautiful women.You are just diminishing your chances of hooking up with them. You could be with a supermodel but you rather approach the ugly chicks because they are the only ones who have  lower self-esteem than you making them easily approachable. Thus, you less scared to talk to them as you feel like you got a better chance. Big mistake! 

I'll repeat this again. You could be dating a supermodel!

Don't be scared. Give yourself a chance.