Historically, in a world of analogue phone lines, spoofed phone numbers were a legitimate feature: if my business has more than one phone line, each of those lines is going to have its own distinct number; but I'm going to want outgoing calls from any of those lines to appear to originate from the same number (the one that I put on my promotional material, invoices, etc.).

Much the same as e-mail, identity within the phone system was, historically, entirely built on trust (because only Ma Bell could make configuration changes, and they could verify that what their customer was asking them to do was legit). Those assumptions were carried through deregulation, and are very deeply entrenched in the technical design of the network. Changing them will hopefully become easier over the next few years as analogue service is turned off entirely (e.g., BT/OR in the UK are set to turn off all analogue phone service by the end of January 2027).