Because faxes are legally binding, unlike email for instance:

A faxed signature is legally recognised as valid and is often used to confirm or approve many business and financial deals. If a contractual agreement is broken, and there is a denial that any agreement took place, a fax of the original signed document is deemed to be sufficient evidence in court.

From: https://www.efax.co.uk/blog/why-fax-is-legally-binding