Why don't team breakaways happen?
Well, it is conceivable that a team could do this on a single stage. However, since the race has 21 stages, it would be an enormous waste of talent and energy.
Every team has specialists: climbers, time trialists, sprinters, etc. Basically, when these guys are in the peloton, they are conserving energy for a later stage, for a sprint, or climb, etc. So, if the entire team made a breakaway, it could very easily sap the energy reserves for a later energy expenditure.
For example... A team breakaway in the mountains would probably mean that the sprinters would be left behind with their energy sapped for an upcoming sprint. Or, a team breakaway on the flats, would mean that a climber would be out of condition for the mountains on the next day.
I could pontificate more, but gotta go....
Back now and some more on the idea of "team breakaways"...
Anyway, my example of a breakaway on the flats was a bad one. Breakaways are very rare on flat stages. Why? Because it's extremely difficult for individuals or small groups to get away from the peloton since there are no big hills to slow it down. However, teams will often work to stay at the front of the peloton so that their sprinter(s) are in good position for a final sprint to the finish.
Breakaways are common on hilly and mountainous stages because hilly/mountainous terrain will filter out non-climbers and weaker riders. A breakaway will be either an individual or a small group of riders, and not, an entire team. Why not an entire team? One is that all members of a team are simply not capable of fending off a chasing peloton for any sustained period of time. For example, a sprinter is rarely if ever, a climber, so you really don't want that rider out in a breakaway. Another reason is strategy. With say a climber and a domestique out in a breakaway, the rest of the team can be back in the peloton manipulating the pace so that the breakaway has a greater chance of success. So, the "team" doesn't breakaway because other strategies are more effective. Another thing to consider is the prize structure of the race. The team classification is calculated by adding the times of the three best riders of each team per stage. So, even if a team was out to win this classification, there would be no point in sending the entire team out on a breakaway.
Theoretically, there could be a good reason for a "team breakaway" in a stage race, but I haven't thought of one yet and have not seen one in the real world. ;~)