Maritime trade is also why major nations build navies and develop colonies and "power projection" - all of which are notably minimized in Civ VI. Battleships are useful as artillery platforms in land battles, because they have a bombardment range of hundreds or thousands of miles. I really can't remember the last time I built an Aircraft Carrier or a Submarine. Of course, from a game-balance perspective, we wouldn't want Civ to model history too closely, because we want a greater range of viable play styles than has existed in history. Strong maritime trade backed by a decent navy has always been "overpowered" in real life. If Civ hewed too closely to reality, the only reason not to pursue a maritime strategy would be if you got saddled with a really poor starting position and had to make do. Still, I think Civ has gone too far in abstracting international trade into a series of passive bonuses, and in making sea trade only slightly better than land trade (iirc, trader units can travel further over water than over land, but they don't yield more). You also don't have to transport luxuries or strategic resources from their point of origin, they just distribute themselves across your empire without effort or impediment.