Most competition bikes weigh significantly more than the 6.8kg limit, because weight just doesn't matter very much. A lot of state-of-the-art road and track bikes weigh around 8kg.
On the flat, weight only affects you during accelerations - at a steady speed, it has no significant impact on performance. Aerodynamic drag and rolling resistance are constantly sapping away power, so features that reduce these losses are nearly always worthwhile even if they increase weight. Even on a moderately hilly road stage, aero trumps weight by a considerable margin; on the track, weight is almost entirely irrelevant, particularly in longer events.
A lot of riders like the feel of a lightweight bike, a lot of them believe that light bikes are faster, but that's only true on exceptionally steep stages or hill climbs.
With the speeds the pro's climb these mountains, aero is still a big deal. For you and me slogging along it's almost all weight that matters. But not for the pros.
Keep in mind that arodynamic drag is not a function of ground speed, but airspeed. If you ride slowly in a relatively windy place, your airspeed can easily be twice of your ground speed.
That said, given how many of us are overweight, worrying about a couple of kilos on the bike is funny talk anyway.
On the flat, weight only affects you during accelerations - at a steady speed, it has no significant impact on performance. Aerodynamic drag and rolling resistance are constantly sapping away power, so features that reduce these losses are nearly always worthwhile even if they increase weight. Even on a moderately hilly road stage, aero trumps weight by a considerable margin; on the track, weight is almost entirely irrelevant, particularly in longer events.
A lot of riders like the feel of a lightweight bike, a lot of them believe that light bikes are faster, but that's only true on exceptionally steep stages or hill climbs.