HVGs are sort of used like cruise missiles, not ballistic missiles. So just because a cruise missile or HGV has enough fuel to go 2500 km in a straight line, that doesn't mean it will be used in such a way. Most of the times cruise missiles (and HVGs) will be used so they go via roundabout routes. A map of enemy presence will be analyzed and a route will be picked where there is least enemy presence and where there is the smallest chance of the cruise missile or the HVG being intercepted. That can often be quite a lot, possibly eating up 30 or 50 percent of the cruise missile's theoretical range. For HVGs it may be somewhat less as they are not powered and they lose some speed with maneuvers, but still, they are MADE to make turns and use those roundabout routes.
So a HVG with a nominal max range of 8000 km is often meant to be used against targets 5000 or 6000 km away optimally. Sure, near the very target, there will be defenses engaging it no matter which direction it comes from. But that's the worst of all places to try an intercept a HGV. It's still fast and it's making terminal lateral maneuvers for evasion. Ideally, one would want to intercept it in its boost phase, when it's behaving predictably like a ballistic missile. But that's most often impossible due to launch site distance. So the only other solution is to try to intercept it during its cruise phase. Against a ballistic missile - that's doable, as it's going straight and is still predictable. As a defender, you more or less know from which direction that threat is going to come from, and you can position your interceptor platforms accordingly - like ships with interpcetor missiles. But a HVG messes with that approach. it flies over a much, much wider area and can go a thousand or two thousand kilometers around your target to approach it from any direction.
So suddenly, instead of having, say, one terminal defence battery that can engage a threat in its terminal dive from 360 degrees but with 20 percent chance of success, and one or two forward defence batteries that can engage the threat in its cruise phase, with over 50 percent chance of success (these percentages are illustrative only)- now you still must have that terminal defence battery but on top of it you need like a dozen more defence batteries to cover the cruise phase threats from all directions. That number can very quickly become hard to deploy and you may simply run out of batteries to do it properly. After all, how many missile destroyers does US have? Scratch 30+ percent for maintanance. Then another high percentage for other missions. And then try to protect several such areas that are scattered around the Pacific. You will not have enough. Or even if you put everything in defenses, you won't have enough left for any offensive ops.