So it's theoretically the happiest day of someone's life. They're having a big party thrown in their honour (in some cases funded by their parents although this is admittedly less the norm than it was). They've got all their friends there, they've found someone they love and everything in the world is right for them.
Why choose this day to give them a really fucking expensive present?
Wouldn't it make more sense to save the present and give it to them on a day when they're really miserable and it'd make a noticeable difference to their mood?