So they started moving to impact weapons instead of close range explosive weapons. Unfortunately that means actually having to hit a target moving very very quickly and dealing with wind, air pockets, etc... The real world success rates of this type of system are woefully unproven. Extremely stacked tests still fail way more than you'd like.
SCUDs and SCUD like missiles are cheap and easy to build, so launching a thousand of them isn't a big deal. Take an insanely expensive anti-SCUD missile with a 50% hit rate (which is generous btw) and you're still letting 500 warheads hit your city, assuming you HAD 1000 anti-SCUDs in the area, which seems unlikely.
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