They look like what we expect race bikes ought to look like.
They instantly evoke nostalgia for simpler times, when cycling didn't focus on marketing of plastic bikes to plastic people.
They have old-world craftsmanship, and are made by people that know what they are doing.
They work properly, for thousands of miles.
They're durable without being overweight.
They ride beautifully, without beating up the rider.
They have fast handling, but are not twitchy.
They often have stunning graphics and paint jobs - but in all honesty, the paint quality really sucks.
They tend to hold their value over time.