When ACLU brings a lawsuit on behalf of an individual, it’s common for the government to moot the lawsuit by changing the treatment of that one person, or offering a settlement to that one person, or even deporting that person, without changing its behavior in general. A class action means the government can’t avoid the lawsuit that way.
What you’re saying is a real issue in general, but not relevant to class actions as used as a civil rights tactic by ACLU.