Originally Posted by Dave Mayer

The (elite-level) teams are actually riding tubulars. Often relabeled. Tubeless has 3 insurmountable disadvantages for performance riding:

• Higher rim weights than tubular rims. This is inherent to the cross-section of the rim: clincher/tubeless rims simply require more material.
• The inability to run at high pressures (yes, higher pressures deliver lower rolling resistance)
• Far greater safety risks in the event of a sudden deflation. On tubulars, the tire stays stuck tight to the rim, and you can ride it for a considerable distance.

Tubeless is a good choice for sponsor marketing campaigns, or for bikes on the roof racks of the team cars, or for the domestiques on flat inconsequential stages.