The odds of someone obtaining credit card information from a plaintext email is a lot higher than someone decoding a VOIP SIP/T38 stream over the public internet, and reading the secrets out of that.
And if its through a traditional carrier (not an Internet provider), its digital, but almost always over their internal network. It is more secure through obscurity alone than plain text email.
I see all sorts of things I shouldn't see when searching the pile of papers on the fax machine at a large company for my fax.
Nothing is 100% safe. It is just a matter of being safer than the effort the attacker is willing to spend gaining hold of the data (or that it will take so long that the data is no longer worth having).
That's not an issue of the security of fax as a transmission method, it's a problem with your company's security at the fax endpoint.
There's also the issue that many fax endpoints that are fax to email gateways. Or fax to non https website gateways.
Edit: Nobody is arguing for using email instead of fax. The argument is to use something better. Like paypal or stripe instead of full cc info + cvv2 on a piece of paper.
When it comes to liability for breaches of confidentiality, yes, it matters very much whether the sender chose an poor channel or the recipient had choose poor practices around what was done with information once it was received.
They are viewed as "safe" for credit card information, as opposed to emails.
Unknown as to why these manufacturers don't provide an online payment system. I work with several that want faxes, and they aren't small companies.
I also saw this with companies that provide drug testing, background checks, etc. Same thing...viewed as a secure channel.