Why couldn't the two Chicago Bears QBs come back into the game if they wanted to or were told to?

If a third string QB enters the game before the start of the 4th quarter all other QBs on the team are not eligible to play for the remainder of the game. It had nothing to do with him entering third either it is because he wasn't listed on the active roster for the game. Teams don't list a third emergency QB as active, but are allowed to use them if they so desire. The loss to the team in using them is that it eliminates the ability to use the other QBs since they are not on the active roster for the game.