Because there is a lot of evidence tied to mailing a letter

• DNA

• Fingerprints

• Time and location the letter was mailed from

• Camera footage of you mailing the letter

• The letter itself can generally be traced back to where it was manufactured

• Address handwriting/stamp/sticker

Even if you scrub anything personally identifying yourself, it still leaves a fairly unique fingerprint. A sufficiently motivated investigator would be able to identify another similar payment without inspecting the actual contents of the letter. Hence pseudo-anonymous