Why service your car?

In December I sent my Polo VW for a service at Volkswagen Autohaus in Centurion. They told me that it is better to take their best service offer of more than R2000 to have everything covered. Over the easter weekend I went away with my family. When I got back there was a weird sound once I began to drive. I then found out that it's my gearbox that broke down because there was almost no gearbox oil in it. I didn't find any leaks of oil whatsoever. When I phoned them they said that they don't check for the gearbox oil because it wasn't the 180 000km service. And only if I asked them to check for it they would have at an extra cost. My question is why would they then have my to take their best service possible and leave out this crucial peace of info? There wasn't a sound then and if there were, it would have been to late for a gearbox wouldn't it?