weight is not everything.
I have a light weight custom aluminium Ridley with 10s Record with light weight Mavic Heliums.
I also have a Litespeed Tuscany with 10s Record and Ksyrium wheels on it.
The litespeed weighs 7.6kg and the Ridley weighs 6.9kg but I prefer riding the Tuscany because it always feels like it's going faster, accelerates better and is a lot more comfortable to ride. It's also a lot tougher. Iuse the Ridley only for very hilly races that are less than 100km. Otherwise the Tuscany always gets the nod.

Pro riders have personal preferences as well and they too are affected by what they believe. if they feel a bike is too light, it will cost them time in mental energy consumed on descents because they don't have any faith in their equipment.

Peace of mind and faith is worth a lot of time when you're sitting in a saddle for 6 hours.

ever sat in a saddle for 6 hours? over hills 1500meters high and 25km long then have to down the other side in a strong crosswind? try it...