The Public Switched Telephone Network is effectively a series of proxies. Some of these are in foreign countries. Each phone companies earns revenue in proportion to the volume of calls flowing through (termination fees). Upgrading their legacy PSTN switches to SIP softswitches that support SHAKEN/STIR costs money. No one wants to invest in a legacy market (voice calls). So they have an incentive to delay deploying countermeasures.