Why can't they defend them individually?
A good worker will have more clout in defending their privileges than a bad one.
At the moment, because of the unions, there is no meritocracy in the public sector (except, perhaps, for a few elite professionals like doctors).
It is too difficult to sack bad workers in the public sector because of the unions. It is too difficult to reward good workers with pay rises and promotions in the public sector because of the unions. Our taxes are too high, because (in part) of public sector unions. Our children get taught by bad teachers, with no recourse, because of public sector unions. People with no pension have to pay for the gold-plated final salary pensions of diversity officers, five-a-day co-ordinators, and all manner of other useless public sector non-jobs.
The only reason to join a union is over matters of health and safety which can have life-threatening consequences. Unions should never be able to bargain over pay and conditions - that is a matter for individuals.
Let's stop our public services wallowing in mediocrity.