I think it's probably just our age. My 6 year old is into Lego and he likes both: making an elaborate set, then after he's bored with it adding the parts to his box of bits and making baroque flying vehicles and rickety 'rescue stations'.

The modern sets are a little less blocky (more curved parts made these days?) but seem to have just as much replay value as when I was a child. Perhaps that's not true of all of them - but then I remember my brother keeping his pirate ship built on his desk for a year or so...