That isn't true. Inertial guidance systems that work for appreciable flight times are too expensive. Many military systems do use GPS, such as HIMARS, Excalibur

That isn't true. Inertial guidance systems that work for appreciable flight times are too expensive. Many military systems do use GPS, such as HIMARS, Excalibur, JDAMs, etc... If I had to guess, the reason why the above are jammed and the UMPK isn't is because of better integration with the launch platform and a shorter flight time leading to less inertial drift over time, plus a very large warhead to make up for inaccuracy.