Because its quite an anomaly. Lets face it, in the majority its really not that hard to make a guy cum. In alot of cases they can cum too soon leaving you wondering what the point of the whole experience was. Some women may feel like they aren't good enough, or attractive enough to you to get you there. Then theres that common idea that if a guy doesn't cum that its frustrating for him, blue ballls and all that crap...
So you don't find it frustrating? Do you masturbate after they have left?
it's worked for me, though you need to be wearing a condom to pull it off successfully.
I am with lonelyguy and 50zcool with regards to the whole not being able to cum thing at times or taking a while.
Just because it sometimes doesnt happen for me or takes a while doesnt mean I am not enjoying the sex. However the woman can have that feeling of dejection etc etc.
You hear a lot women complaining about sprinters, it can be just as frustrating and annoying for them when they are tangled up with a marathon runner.