Thats the whole point, if you watch you'll see that the teams send somoen up the road to make a break to see if anyone will chase him down, usualy unless they seriously think he can get away and stay away they will ignore him and let him suffer on the front, and all the rest of the team do is protect their main sprinter and bring him to the front in the last mile or so.
The only time its gets difficult is in the mountains because sprinters usualy cannot climb, unless you are some kind of cycling god like Eddie Merckx.