Can't that be faked in the same way?

Many telephone scams rely on creating sustained panic in the victim. Transfer your money to thwart the cybercriminals! There's no time to think.

A letter can be discussed with friends and family. It's much easier to dismiss without a con artist whispering in your ear.

Most cyber criminals have a script. They don't deviate from the script unless they think they have a potential home run. Even then, going from this to sending out fake mail correspondence... That's a whole different toolbox. 99%+ of the time they will not even consider it. Especially since it's not scalable.

I remember that scammers got in touch with my wife, trying to get personal info. It was fairly elaborate. She got a call from a man that she said had “a golden voice,” followed by official-looking mail correspondence (very quickly, which was suspicious, in itself -it can take many days for my bank to get me correspondence). They had our home phone number, her name (not mine), and address; either through public records, or via a breach (which is why “they didn’t get customer credit card info” is a worthless reassurance).

It was “Synchrony Bank,” telling her she was victim of a fraud. I contacted the real Synchrony Bank, and let them know about the fraud. The contacts stopped.

Sure it can be faked.

But but it'll filter out most of such scams which are "online-only".

But it is much costlier (time, materials, logos…) and a different technique.