TeMPOraL parent
As others have said - the primary reason is that you need 1+ nukes to take out the other side's launch sites, and the other side knows this so they spread the siloses across their whole territory - so now you need 1+ nukes for every silo they have (or suspect they have), and they need 1+ nukes for every silo you have (or suspect you have), and this is a positive feedback loop.
By the time your nukes get to the silos, they will most likely be empty already though.
Which is the reason you want to have more nukes than they have silos - to be able to hit all of them at once in a surprise attack (also called "first strike").
Which won't save you against their SSBN… Granted, they have much less warheads in submarines than in silos, but that's still far enough to obliterate every city above 100.000 inhabitant in your country.
Indeed, which is why submarines are the linchpin of modern MAD. But they weren't always present; the buildup of nuclear arsenal started when the options were limited to ICBMs and strategic bombers.
Efficiency of submarine fleets were always hard to calculate, based largely on guesses about the adversary's true anti-submarine capabilities.