On top of that, there's an assumption that there's no existing cleaning effort. I'm sure there is and it's just a difficult problem. The cases left must be either in progress, hard to track down, or not actually meaningfully active.

Or, as is really common with the federal government, the agency is actually underfunded and hasn't been able to modernize because the Republicans in congress have been trying to starve the administrative capacity the classic, slow way until now.

Like with the IRS. I've made mistakes in filing, and gotten a notice from the IRS about it, but sometimes years later (!). In the meantime, if you "audited" the IRS records, you'd see that my records are out of compliance and could claim "See, there's fraud!". In reality, the IRS just has slow antiquated systems, and is barred from giving taxpayers direct access to their records. Which is by design from the rich and anti-government.