Actually, a lot of the difficulty comes from regulations put in place because carriers were shit.

In one of the first cases involving Net Neutrality, the Madison River Telephone Company (now CenturyLink or whatever they've changed their name to this month) started blocking Caller ID from VOIP sevices, and eventually blocked Vonage outright.

The FCC came down hard on them, and in the aftermath one of the things they did was essentially prohibit carriers from blocking CallerID spoofing, because there's a lot of legitimate uses for it.

For instance, when I can load up the VOIP client on my cell phone to make a work call, I need the CID to show my office number. Or if someone works at a call center, their PC dials out with a phone number that's the switchboard number for the center.