It wouldn't be impossible at all to pull in locations of all ships and render them, though.

One could do a pretty good job of it too. AIS data contains position, heading, and speed. So it's everything you need to interpolate movement between data updates.

The only problem being, nobody provides AIS data for free. Though I wonder how much a subscription would cost for personal use.

The game struggles to render everything enough already.

Is rendering ships on approximately real world courses more rendering work than rendering them on pseudorandomly selected courses? I don't think anyone is suggesting the game ramps up the density settings or pulls in high res models for them

Yes. The stock ships may be scenery objects running in optimized positions/places. Realworld data would not be optimized.

Isn't that what spatial trees are for?

Ooh do planes too.

https://flightaware.com/

I was under the impression the newest MS Flight Sim does have all the planes flying around according to real data.

Yes, real-time aircraft are in the latest MSFS based on their real-world movements, with correct callsigns and flight numbers, if you turn the option on for real-world traffic.

I'm not sure about all, but it definitely incorporates real-time plane tracking.

The new MS Flight Simulator uses Bing Maps imagery so I originally thought the satellite map happened to include the ship and thus the game would've had it as well, at least in "flat" form as a terrain texture.

"Just a mod" is a bit of an understatement I think, as I understand it, the FS2020 core product is essentially a "hub" for 1st- and 3rd-party mods.