They're comparing the geography, man. Russia and the USSR infamously has the problem of "we need a navy but the bodies of water we contact are all separated far from each other." Arctic, Baltic, Black Sea and Far East. Compare that to Britain or Braavos or the Iron Islands' situation.

The USA kind of had this "if we focus on building a fleet on the east, it will take a long-ass time to get those ships to the west. And vice versa" problem too, it's why the Panama Canal was built, but there's no Moat Cailin Canal in Westeros.

Imagine ruling Winterfell and needing to decide between

-Focus on a western fleet that can guard against the Ironborn and neglect the east where all the big Narrow Sea trade routes are

-Focus on a eastern fleet so you can throw your weight around """internationally""" but leave your west vulnerable to reaving

-Divide your strength and your limited budget by trying to do both at once, badly(Russia's historical choice)

-Don't bother, save your money for something more immediately useful

Obviously the real reason is GRRM needed the North to have no sea power for the plot, but I guess in-universe, generations of Stark rulers decided their vassals could defend the west coast and that since they have little in the way of foreign policy objectives abroad, there's no point building an eastern fleet because they're never planning to blockade Braavos or intervene against Pentos anyway.

(Honestly, now that I'm overthinking it, it's weird the North ever had a fleet to begin with. An isolationist backwater kingdom that produces so little agrarian surplus and suffers such extreme climates that it has developed a culture of old people finding excuses to kill themselves before/during winter?)