1) Why is spoofing still possible?
2) How dumb are criminals: "according to ads for the fraud service on Snapchat, Instagram, and Telegram"
It's really not hard. Override any requested originating number information and reveal the originating phone number unless and until they prove ownership of an alternative they wish to use (e.g. a shortcode name like NHS, or a main switchboard number).
We can do that for landlines and mobiles overnight.
Hell, make it a subscription service. Want to use another number than the one you're calling from? £5 a month on your bill, and subject to checks that you own that number.