Because cycling, stage races at least, are made up of several individual races. There is of course the general classification at the end, which especially in terms of media exposure and prize money is the main thing, but that part is basically only for the strong climbers. There's plenty of other riders, with different specialties, who all have their chances in individual stages (not in the sense to give them something 'because otherwise it's unfair', but because a good stage race should consist of many different types of stages, from long and flat to short and mountaineous).
A podium makes sense, like described in the post above, a stage is kind of like a Formula 1 race, with the final classification being the championship. It's not really like in rally, where you have 5+ stages a day, and the whole rally is done in a few days. Stage wins are counted, but not specifically celebrated there.
The bottle of champagne thing...that's just sponsors I mean, a podium does need something, because it's an actual win in a semi-separate race, but the bottles being so big is just a sponsor thing.