Because the script didn’t start life as a Halo show. It probably started life as an original show that the producers/show runners couldn’t get funding for. Then some had Halo dropped on their lap and adjusted the script to be aesthetically Halo.

The fact thay Halo is well known for taking common tropes and subverting them subtly makes this plan work on a superficial level, but fall apart on a deeper level. In the books Halsey specifically stated that she wouldn’t brainwash the Spartans because if the brainwashing broke, they would be more dangerous than the insurgents they were trained to fight. Something the tv series missed.