The problem is that there are a lot of spurious and dubious cases, and class-actions are very expensive and risky to defend, even when the defendant is innocent

The problem is that there are a lot of spurious and dubious cases, and class-actions are very expensive and risky to defend, even when the defendant is innocent, largely because they're seen as unsympathetic 'piggy-banks' (whereas the plaintiffs are often very sympathetic).