most triathlons are not the ironman. sprint distance is 20km on the bike, olympic is 40 km and ITU long distance is 80 km. all much shorter than road races.

tri bikes are also set up to put the rider in a very aero (usually uncomfortable) position. reasons for this include (relatively) short distances, energy conservation for other legs, and the fact that in most amateur tris you're not allowed to draft and work as a team against the wind.