Boxers from light middleweight down to light weight divisions are calling out Pacquiao but not Mayweather?

From what I have been reading, Foreman and Paul Williams who are both much taller than Pacquiao want to fight Manny Pacquiao but I have yet to hear them call out Mayweather who is near their height. And now Berto with his recent win is calling out Pacquiao but not Mayweather. Even Bradley, Alexander and Malinaggi, Juanma Marquez, Valero, and recently resurrected Erik Morales are all saying they want to fight Pacquiao, but no mention of them wishing to fight Mayweather. Are they all afraid of Floyd Mayweather? Or they all consider Manny Pacquiao as the Top Dog to topple?