You are missing point...
Today has surely cost something, even if they didnt attack each other. It sure tired some people out. Both some of the captains and domestics, that might just had enough to stay with the group today. That might drop on Saturday for just a small shift in pace up Port Bales for example. Saturday was always gonna be the real test this week to see who is still in contention going into 2nd week.
To put someone into red today you would probably had to put yourself into red, a maximum effort. For what? 20s. Makes no sense. They are managing their efforts to have the most energy where it will count the most. No one is truly that much better than anyone else that they can just attack and attack. It costs too much. Easier to just have them drop on a stage because they gone into red so many times to just stick with.
Broccolidwarf has a point though.
If Bernal or Roglic is hurting now from their Dauphine injuries, or Pinot from his crash on stage 1, then to respond to attacks now will be an effort that will cost them on Saturday and Sunday, when riders on other teams will have to attack them.
If Bernal and Roglic don't have to respond to attacks now, they will be better rested to deal with those Saturday and Sunday attacks.
By not attacking today, whichever GC riders were planning on attacking on Saturday made their attacks less effective. particularly, say, Trek or EF, who have 2 or more riders still within a minute of yellow, who can afford to burn one.