After a lot of research on this topic over the years, Naval Supremacy and Eurasian Encirclement is the best answer I can come up with.
As an extension of Halford John Mackinder’s “Heartland Theory,” upon so much of which modern military strategy is based, the “Rimland” is an inherently naval conquest:
This is in contrast to the Heartland, which is a land mass at the center of Eurasia. NATO buffers the Heartland to the West, but as Eastern trade routes are largely seafaring and there’s no unilateral chokepoint (a-la the Strait of Hormuz, Panama Canal, etc.) the US Empire needed to control a lot of open water to limit to monopolize trade in the region.
The Korean War was the USA’s first stab at control of a huge swath of the Rimland - effectively bringing the entirety of the Sea of Japan under their control. Vietnam was an attempt to do the same for Southeast Asian water passages, in addition to key natural resources for black markets (namely opium).
It’s no coincidence that China’s recent “New Silk Road” proposal would have to make liberal use of these waters to be successful - geopolitical machinations are a long game if there ever was one.
Globalist shill Zbignew Brzezinski outlined the extended version of Mackinder’s Heartland in his 1997 book, “The Grand Chessboard.” It’s a very good window into the mind of a Global Imperialist.
I took a stab at analyzing this long game perspective a few years back - I can’t say I still believe everything I wrote in this article nor is it directly about the Korean War, but the broader geopolitical agenda extends through to the present: https://www.activistpost.com/2016/08/cia-breaks-silence-new-silk-road-says-challenging-brics-aiib-mistake.html
A complementary theory is that Korea was used as a slaughter to legitimize the United Nations as “global peacekeepers” despite Truman himself allowing the installation of Kim Il-Sung post-WWII, setting up conditions for the “official” story: https://jamesperloff.com/tag/korean-war/
I’m not sure there’s any good single reason, but I’ve always found it fascinating and revolting that South Korea was little more than a competing military junta for decades until the 1980s. Most Westerners tend to forget (or never learned) that.