Putting silos too close together certainly makes no military sense, but they are cheaper to build that way.
Or perhaps the idea was to get the enemy to strike repeatedly at a bunch of empty silos... Or more charitably, maybe a lot of the silos were dummies, to force the enemy to expend a lot of expensive silo busters on empty target silos.

Nuclear war fighting capability can be defined as offensive, or first-strike, or defensive (second-strike or retaliatory/deterrent). The former are highly accurate and expensive weapons produced in relatively small numbers since they will be used to start a war. They are aimed at hardened or highly defended military targets. The latter are cheap, not particularly accurate, but produced in large numbers so enough will survive a first strike. They are aimed at cities.

Then there was the Reagan plan to convert the sub-launched Polaris ICBM force (a perfectly reasonable retaliatory defensive configuration) into a super-accurate MIRVed first strike system. The idea was the sub-launched missiles would take out Soviet air defenses so our bombers could get through easier. Why not just let the missiles replace the bombers altogether?

It turns out the Air Force was afraid the bombers would become obsolete, and the Navy wanted to get funding to upgrade its obsolete sub force. Unfortunately, it converted a perfectly effective and credible retaliatory defensive system (one that could be held back in reserve until the nature of the attack was perfectly well understood)that already existed into a highly destabilizing first strike weapon that scared the hell out of the Soviets.

A lot of these "defense" decisions have little to do with national security or military necessity, they are political or economic moves made to satisfy local state interests, or economic centers, or even inter-service rivalries.

Several times during the Cold War both sides almost precipitated a conflict by proposing or even deploying systems of dubious deterrent value but obvious aggressive (first strike) capabilities. The decisions to introduce these systems were usually political or economic, not military.

Other systems, like Jimmy Carter's idea to put our 2nd-strike missiles on rails so they could be shuffled around (an excellent way to make a Soviet first strike harder to pull off} were dismissed as ridiculous.

Instead, we went for replacing the perfectly good second-strike missiles in our hardened silos with highly accurate Mirved first strike rockets. I personally worked on one system designed to frustrate and bypass a non-threatening Soviet retaliatory system to make our own first strike easier.

Look up Hermann Kahn.