As Ustwo said the contractions would be hard to pull off. There is also the time spent with your partner. Sure I might miss it early on but once I get to know the girl it's pretty easy to spot the fake from the real, there are simply too many singnals involved. Half the time I know how well I'll do before remove the panties. Then there is the way you girls act afterwards.
Also, just because you ladies think you can fake it, doesnt mean you don't get caught.
Sometime us guys simply accept the faking. Not all the time mind you
There are simply times when you fuck up on the technique or come on too strong or she accepts just to indulge but isn't really in the mood for it. Hell it happens. It's an easy out for you, but it's also an easy out for us. So we accept it.