Because, a) “people” don’t like time trials (don’t ask me which people) and b) if you put 100km of TT in any TdF route in the past decade, it’s basically giving Sky (be it Froome, Thomas or Wiggins, but not Bernal so much) a 2 minute head start over any other realistic GC contenders, until the emergence of Roglic and Dumoulin.
Are TTs really more boring for, let's say casual fans than flat stages? I feel a TT heavy GT once in a while would be an exciting change, a prologue, a long TTT, 2 ITTs. Maybe it would even create a more animated race in the mountains, forcing pure climbers to attack earlier, not waiting for the last few km to gain 20 seconds.