I think the likely reason is they have signed a non disparagement agreement and have no avenue to publicly criticize. Additionally, quietly criticizing just means quietly getting fired, then likely still being unable to publicly criticize.
The only way to say "hey, I don't think this is right and I don't agree with what's happening here" is to publicly resign and hope that alarm bells start going off in people's heads as to why many of these folks are resigning simultaneously.
US Government employees basically can't sign non-disparagement agreements, the Federal Government doesn't enter into agreements without a fixed time commitment to judge compliance, and there are incredibly strong whistleblower protection laws for Federal employees which would make any non-disparagement agreement very difficult if not impossible to enforce. A Federal judge held that the Trump 2016 campaign's non-disparagement agreements were unenforceable, and there have been no hints of Federal employees being offered anything even like that.
That bit of pedantry aside, I agree with you that the purpose is to draw attention to something bad happening, it is a grander version of leaking to a reporter.
> I think the likely reason is they have signed a non disparagement agreement and have no avenue to publicly criticize
That was litigated during Trump's first term and held to be not enforceable. That was the case brought by one of his reality show contestants that he appointed to something (the exact details are too trivial to care about).
The only way to say "hey, I don't think this is right and I don't agree with what's happening here" is to publicly resign and hope that alarm bells start going off in people's heads as to why many of these folks are resigning simultaneously.