Howcome all the good motorcycle manufacturers don't have to advertise in any way to sell their bikes?

If you live in the US you are missing the big picture. These particular manufacturers don't have a big presence in the US which is why they don't spend much in the way of advertising dollars in the US. Yes many people buy these bikes in America but I don't think we are their main target audience. How many Moto Guzzi's do you see on our roads compared to Honda's? Some Americans buy Lamborghini's also but you don't see their commercials in the US. I'm sure you do in Italy though. Good go at trying to make your chosen brand seem superior though...typical.