America can't shoot down hypersonic manouvering missiles... just like the Russians and Chinese can't either.
A hypersonic falling item like a Scud or other ballistic target can be intercepted because its flight path is predictable... for an object flying at hypersonic speeds that is not ballistic and could go anywhere things are much much harder...
Imagine an incoming Zircon missile moving at mach 8... which is 320m/s x 8 = about 2.5km/s...
If you detect the target 100km away, you need to calculate its trajectory... its expected flight path... so with computers and three pings from the radar lets say that takes 4 seconds, so the missile is now 90km away, so you launch your missile to intercept and you vector that missile to hit the target... it will take your missile 25 seconds to get to the interception point because it needs to accelerate and climb... so that means the interception point will be at about 30km from your launch platform for the SAM... (the incoming Zircon will cover about 62km in the 25 seconds it takes your missile to get to the interception point... but hang on... unlike a ballistic missile... the Zircon can manouver in 3 dimensions... it can climb or descend and turn left or right... and it can speed up or slow down... in which case the interception point can change in a second and suddenly be several kilometres away from the original interception point.
The problem is that any SAM launched it heading towards the old interception point and to now turn and head to an new interception point uses up energy... which slows it down... the target is still coming at 2.5km per second and that first missile might not make it to any interception point in time... it is not just a case of getting that interception missile to the same 3 dimensional point in space... it also has to be there at exactly the same time... half a second either way and you miss by over 1km... no amount of HE will be effective with such a miss distance.
More importantly they will have programmed the missile to manouver to be difficult to hit... it is actively trying to avoid any missiles fired at it, though it has no way of actually detecting anything coming at it... unless they have made the radar antenna in its nose sensitive to ARH missile signals... it wont be able to track them but detection would warn the missile to manouver more... it is scramjet powered so it is still powered so manouvers wont reduce speed very much at all anyway.
The Granit has titanium armour to protect the warhead from incoming Phalanx rounds... during testing it took two AA-9 missile hits to bring one down and it only moves at Mach 2...
For intercepting missiles detonating the warhead is your best chance of defeating it... which is really hard if you can't hit it.