4X games usually don't model trade networks beyond bonuses to money and access to some vital resources. Every city is a self contained autarky. And that's fine, 4X games are so abstract that setting up a game system where having a "super science/production/commerce etc city" has to be fed by a "food city" would be very clunky and not fun at all. Not to mention 4X AI can barely handle land combat, never mind balance the complexity and defenses of self-sufficiency vs specialization.> it's also insanely easy to starve out a nation by blocking their naval trade in real life
Every nation in real life (that you would think would be vulnerable to this) today has insane protectionism to their farm sector to prevent this sort of thing. USA, China, Russia, all the European countries, they're all actually self sufficient in food staples. Britain in WW2 would had survived perfectly fine with rationing, even had the Battle of the Atlantic not swung in their favor. In WWI the blockade on Germany was a contributing factor but the real cause of food shortages was due to the lack of men working on the farms due to war demands (A thing that 4X doesn't model).
tl;dr You have a bunch of misconceptions about blockades and it's all well and fine to complain that oceans don't reflect reality, but 4X games just don't work like that by default, unless effort is actually put into that particular part of the game, which I don't think has ever happened.