There are two main reasons:

1. Hills: Pushing a light bike up a hill is obviously easier than a heavier bike. Everyone likes to brag about that HC climb he did the other day. Without resorting the lying about actually climbing the hill, the easiest way for a cyclist to make his life easier is to carry less mass up the hill. Nobody will say, "That didn't count because you rode a 3kg bike," and you get no extra points for lugging a 40 kg dutch commuter bike to the summit.
2. Technical Competence: Bicycle manufacturer have tried over the years to reduce bicycle weight, to the extent that weight (or lightness) has become a proxy measurement of the manufacturer's technical competence. Anyone can build a heavy bike, but it takes real engineering skill to build one that is light.

There are two main reasons:

1. Hills: Pushing a light bike up a hill is obviously easier than a heavier bike. Everyone likes to brag about that HC climb he did the other day. Without resorting to lying about actually climbing the hill, the easiest way for a cyclist to make his life easier is to carry less mass up the hill. Nobody will say, "That didn't count because you rode a 3kg bike," and you get no extra points for lugging a 40 kg dutch commuter bike to the summit.
2. Technical Competence: Bicycle manufacturer have tried over the years to reduce bicycle weight, to the extent that weight (or lightness) has become a proxy measurement of the manufacturer's technical competence. Anyone can build a heavy bike, but it takes real engineering skill to build one that is light.