That's probably because male porn performers have sexual access to so many female performers, that they don't think to enjoy each one properly, like regular men would do in real life. Perhaps also they don't even like some of the women and so they just do the minimum sometimes, to make the film OK.
why do these men not suck her nipples??? i hate these pathetic men who only squeeze titties and do nothing for the women ... what about sucking her clit