Nuclear war planning isn't as simple as applying a rate of interception or failure to your stock of warheads. You have to plan for eventualities like what happens if you're subject to a first strike - can you ensure that enough of your own warheads will survive to retaliate? If not, or if your opponent thinks not then your opponent is much more likely to try a first strike.

Modern missiles aren't just faster or harder to shoot down, they're also more survivable. Have you noticed that while the Russians and Chinese parade their missiles on big ass trucks, the US doesn't seem to have any? That's because there isn't a road or rail mobile variant of the Minuteman 3. So those MM3s have been sitting in silos only for decades, more than enough time for opponent satellites to pinpoint exactly where they are. On the other hand, a Russian or Chinese missile can drive around their own road or rail systems and be untraceable unless you have real time satellite footage that just happens to catch them moving.

So if your missiles can't move, you can only protect them by hardening their emplacements and silos. Unfortunately, most American silos are about as old as the missiles in them and were designed to withstand much lesser yields of warheads. Maybe some could be brought up to a newer standard, but building of that scale would also paradoxically tip your opponent off to which missile sites to target first.

Therefore, if you're in a position where you aren't convinced your own missiles will survive a first strike, your only move to maintain deterence is new missiles or more missiles (or both). Contracts were passed out for new missile designs around 2017 but it seems like nothing has come to fruition. Therefore the only other option is to build more warheads so that they can be fired from planes and other systems instead.

This leads on to the next point which is that warheads are not all necessarily sitting on missiles read to go at all times. Most of the time they're in central stockpiles that are easier to guard and maintain and are only parcelled out to units in times of heightened nuclear tension. A modern nuclear power has more platforms that can deliver nukes than actual nukes themselves - the whole point of a nuclear triad (ICBMs, planes, subs) is to ensure maximal redundancy so that no one type of attack can destroy all delivery systems.

Hence, a nuclear war planner has to figure out how many ICBMs and warheads are likely to survive a first strike, then figure out how many warheads are needed to put on planes and ships and subs for a counter strike. If the US military is thimking it needs more warheads, then one major reason could be that it's realized it's delivery platforms are not as survivable as predicted.