Tax data is also way more sensitive than just SSN, as it includes details about salary, investments, property ownership, marital status, what organizations one supports financially, etc.
Treasury payments also go to companies, including companies engaged in highly sensitive projects like classified weapons systems.
This is all just off the top of my head as someone who is somewhat familiar with the federal government. The article is worth a read for more details.
Typically security of this data is aided by compartmentalization; no one has access to aggregate and collate it all. But now random people do.
And typically the people who secure and work with this data are carefully vetted to reduce the chance they are disgruntled, blackmailed, etc. But now it’s a bunch of random young guys with no clearances or background checks.
And usually many of these systems are air gapped from the Internet. But now they are being accessed with consumer laptops off the street and data is sent off in clear text to commercial data centers where it is likely logged, copied, stored.
It is a real problem that is being swept aside through political signaling. But the real consequences will likely bother Americans for a long time.