The USA is particularly strong from that side. Most of its navy patrols those waters, many islands on the Pacific are USA territories all the way to Japan which itself acts as an arm of the USA Military since the forced demilitarization of Japan in WWII. Japan and other islands basically form a wall around the eastern mainland that would be difficult to cross undetected, and when they did reach the shore they might get flanked from Hawaii, Panama, or Alaska.

There is a reason Stalin chose to ally with and provide nuclear weapons to Cuba, and approach the USA via the gulf of mexico and Atlantic.

The Americans are the premier naval power.

Hawaii, Midway and other Pacific islands having monitoring stations.

The Pacific isn't as easy to cross as you think due to it's size and it has some nasty ass storms.

Pair that with the coastal regions usually having cliffs rather than somewhere a landing craft could pull up and it isn't easy for a start.

Now mix in people who know the terrain and don't want you there.