When one has a special needs child, for whom services were provided through the public school system, that is not always a viable option. Never mind those parents who have to work to put food on the table, but the costs involved in replacing those services can be astronomical...and that doesn't take into account that finding services outside of school can be extraordinarily difficult and more often than not involves lengthy waiting lists.
ETA: And in many, many cases, parents have fought for YEARS to get the appropriate services in place at school for their children. Removal from the district would mean that they'd have to fight all over again should they decide to re-enter the public school system.
EDIT 2: After speaking with both my sons' teachers, the only students they had difficulty with in regards to masking were those children whose parents were obviously anti-mask.