Because it's a team sport. When you're in the break, your team's entire possibilities are in you. 4th place might be a nice finish for an individual, but for a team, it's only a little above average. Don't think of it as 4th out of 189 riders, think of it as 4th out of 21 teams. Then think about the fact that the winner gets 10 times the coverage for the team and sponsor as 2nd place. Your job in the break is to win, and you go all or nothing. If the break does stay away and you come in last, it's going to be a quiet bus ride back to the hotel.