Because the only way to do that is put yourself in the crosshairs. If 3 of the 20 companies are being sued, the other 17 could ask for a summary judgement and a

Because the only way to do that is put yourself in the crosshairs. If 3 of the 20 companies are being sued, the other 17 could ask for a summary judgement and allow everyone to pool resources, but then 17 companies had to put the result of the lawsuit on their own heads too. You are correct that it would be better for the system as a whole, but each individual is better off not participating.