The reason spoofed Called ID works is the same reason that we get spam emails, the same reason that Blue Boxes worked, the same reason that Citizens Band radios used to work and are now almost all useless.
We don't count costs much in America. Whenever it's possible to "improve" a system or a service, we go ahead and do it, without asking ourselves if it's a good idea or if the benefits justify the risks.
I don't know why. In the case of robocalls and spoofed Caller ID, perhaps it's the result of the telephone network having been, for almost all its history, a closed system that had no outside competition for ideas, no internal or external process to guard against unexpected results, and no meaningful standards of security.
Maybe it's because technocrats such as we have a kind of tunnel vision that prevents us from seeing anything but the shortest path from a "problem" to a "solution." Our collective consciousness does not allow for suspicion or caution - just the need to solve any problem we're asked to address.
I doubt it ever occurred to the engineers at Bell Laboratories that allowing customer-provided-equipment (CPE) to supply Caller ID info was an invitation to fraud. I doubt that their world-view allowed them to even consider the posibility that someone would break their rules for commercial gain.
I don't know the answer, but I think we need to consider the question before we set about trying to "solve" a "problem" without thinking if we should look behind the curtains of American business and ask ourselves if the "cure" is, once again, worse than the disease.