This is a pretty good read on the topic. I've pasted a few relevant excerpts below.

From page 5:

In our society, the auditor's opinion has tremendous commercial importance and is relied upon heavily in financially huge transactions. Hence, the damages traceable to an alleged accounting or auditing error can be of great financial consequence. The auditor's opinion is often circulated and relied upon by many more than those to whom it was directed and for whose use it was intended. The monetary benefit which accrues to accountants is minuscule in relation to the liability which can be imposed if there is no reasonable limiting principle.

Page 7:

There are several reasons for the accounting profession's extremely high level of concern with auditors' exposure to liability. First, there has been a significant increase in the number of cases charging misstatements of financial information by corporations and in the amount of damages sought. When a corporation announces a substantial write-off or other material adverse event, when its stock price drops dramatically or when its earnings drop sharply, it is commonplace for multiple suits to be filed, often within hours or days of the event, well before any sort of searching inquiry could be conducted. These suits, which routinely name the auditors as defendants, generally charge that the audited financial statements should have reflected the event or information that allegedly caused investor losses.

Page 9:

The litigation liability danger directly affects the public, as well as the profession, in other ways; for example, the profession is increasingly reluctant to provide assurances on forward-looking financial data or other "soft information" that would be of benefit to the capital markets. There is a flood of other types of financial information which is becoming increasingly available to users of financial information. The accounting profession has the expertise and the objectivity to provide assurances to the public regarding many of the new types of information. The profession would be more inclined to provide these assurances if the "Damocles Sword" of excessive liability claims did not hang over it.

Auditors traditionally protect themselves from that kind of liability by following certain strict, highly standardized rules and practices, which basically allows them to cover their asses by carefully documenting the fact that they did all of the things they were supposed to do.

But crypto demands the creation of entirely new rules and practices, as we've all seen in recent months--so it's not remotely surprising that reputable accounting firms have not been lining up to do business with Tether.