I mean it's a nuclear missile, millimeter accuracy isn't really necessary. Somewhere in the general vicinity is good enough for it's purpose of going boom.
Well, accuracy makes a big difference if you're trying to hit a hardened target like a missile silo. Missile guidance has been a constant effort to squeeze out more and more accuracy. Minuteman I started with an accuracy of 2 km, but now Minuteman III is said to have an accuracy of 120 meters. The Peacekeeper (MX) missile, no longer in service, is said to have an accuracy of 40 meters. You can use a much, much smaller warhead if you're 40 meters away compared to 2 kilometers.
The START II treaty limited Russia and the US's ICBMs to a single warhead each, and the Peacekeepers were optimised as a platform to host multiple independently targetable re-entry vehicles (MIRVs) and when the US agreed to revert to a single warhead per missile the Minuteman III was much cheaper to maintain than the Peacekeeper.
So even though Russia withdrew from START II almost immediately, the US continued to unilaterally remove the MIRV capability from its ICBM fleet and stick to single warhead Minuteman IIIs.