Actually since they follow a predictable path and don't take evasive action they are easier to intercept than bombers. And before anyone says that ABM systems don't work the Americans were able to get skin to skin interceptions of ballistic missiles back in the '60s.
Conversely bombers can use jamming, take evasive action when attacked, hide behind terrain, or weather and approach from unexpected directions, none of which a ballistic missile can do.
Even without land based ICBMs there are still lots of sub based SLBMs, cruise missiles that can be nuclear armed, not to mention all of the strategic bombers that the USAF and the Russian Air Force still have.
Well, the fact that no one in the world has deployed a big, viable ABM force while people have been intercepting bombers almost since they were designed points against that...not to mention that missiles
can and, but for the fall of the USSR and various treaties,
would take evasive action...look up "MARVs" if you don't believe me. Or is screaming in at high Mach numbers less than a few thousand feet over the ground after reentering some distance from your target not evasive action?
Beyond that, of course, there are penaids like chaff, decoy warheads, jammers and the like, or FOBS, which can deploy weapons into LEO to hit the enemy from any direction...modern ICBMs also have a long enough range to just go over the South Pole (or take other unexpected action) to hit their targets, too, rather than having to go via the North Pole. So, it is just not true to say that ICBMs, unlike bombers, must march methodically towards their destination, taking no evasive action whatsoever or attempting stealth, and are thus easy to intercept.