The problem is probably the fact that with current technology it's much easier to make a missile go faster than to develop a computer capable of tracking those missiles, a radar capable of keeping a lock on them despite evasion and stealth, and an interceptor capable of downing them reliably.

Missiles and engines are just improving on existing tech, which is always faster and more reliable than innovating completely new tech. By the time the US develops something capable of intercepting Mach 6 missiles, Russia and China might have Mach 8 missiles in service.