School Districts are mandated to provide the number of days/hours of "teaching" set by the local District or the State.
If teachers go on strike the school year is extended to provide the students with the required "education". As a result the teachers suffer no consequences as a result of their strike. Strikes are relatively rare or if at all short lived in the private sector unions. Strikes generally cost the workers more than they can makeup in a new contract.
If teachers actually suffered some loss they would be less prone to strike.