Even in a TTT somebody still has to ride into the wind (even with a tailwind) for a reasonable amount of time. They could push a bigger gear and go faster, but they'll have to drop to the back sooner, which puts TWO riders riding into the wind during the transition. A 53/11 will propel you around 39 mph at 100 RPM or 46.5 mph at 120 RPM. If you're already going almost 40mph, it's either a long descent, or it's a good time to rest and save energy for the next time you WON'T be going 40mph.

The only instance I could see a bigger gear being useful is in the case of a combination of factors; a one-way, downhill, time trial with few turns, and a monster tailwind. Cancellara ran out of gear at Tour of Qatar this year when he hit almost 50mph on the flats due to monster tailwinds.