They can. They have little interest to. Very miniscule reward (the only ones winning in a class action are the lawyers... I am due for a solid $20 from Google though!) for a huge risk.
I'm repeating the words of another commenter, but most app devs aren't competing against Apple but other devs. They aren't billion dollar businesses that stand to benefit by trying to get a lower rev share.
But you've just described the dynamics of every single class action lawsuit. It doesn't matter that the individual devs have little interest, the lawyers have huge interest because winning a sizable class action for a lawyer is equivalent to a having a startup that hits.
The purpose of this class action would be to force Apple to change policies going forward (which would presumably give choices to developers that would allow them to save money and increase their profits). Any cash distributed in the settlement would be a bonus.
But I do agree that most developers probably wouldn't be interested. They don't want to stand up their own payment processor, or don't care to do integrations with a bunch of third parties, and tolerate the fees to use Apple's payments platform. And for many of these developers, their entire business is built on their relationship with Apple. Getting their developer accounts terminated would mean shutting down entirely.
I'm repeating the words of another commenter, but most app devs aren't competing against Apple but other devs. They aren't billion dollar businesses that stand to benefit by trying to get a lower rev share.