When Rukia was explaining how Mod Souls were created for Project Spearhead, she said the project was scrapped because forcing corpses to fight was immoral.
But why weren't Gigai considered? Unlike corpses, Gigai are created so they are more readily available and a Mod Soul would inhabit a corpse in the same way as a Soul Reaper inhabits a Gigai with the exception that a Gigai was never alive to begin with.
So why wasn't it considered to use Gigai instead rather than scrap Spearhead altogether?