Simply tradition I fear.
But it could have to do with having ridden 3500 kms and being a bit tired too.
Like Lemond perhaps one year someone who really wants it will break the unwritten rule and go for it.
Problem is 8 laps of the Champs Elysee. Probably the best you will get is 1 lap lead and the teams will seriously chase that down. So who knows.
The Major
Clean is good