Logistics, and detailed military considerations aside, the key concept to take into account is US security: there are economic and political influence benefits from having significant allies in Europe and globally (and, to some extent, counteracting the waning global projection capabilities of Russia).
Many of the European allies are however not entirely committed (i.e. consider their typically < 2% GDP defense budgets) to maintaining armed forces and the US is running a calculated operation balancing out its costs vs benefits. Without a strong US presence, NATO might not exist in the same form and the US may not benefit as much from having them, and the EU, "in its corner".
Why the United States Still Needs Ground Forces in Europe | RAND
The presence of U.S. forces draws additional critical, albeit more abstract, benefits. The scale of U.S. forces on the continent is a key ingredient to U.S. influence in NATO institutions. Without the glue of U.S. strategic thinking to anchor the alliance, NATO would probably struggle to develop coherent military policies and plans. If the United States were to diminish its own contributions, it would also undermine the message the United States is currently giving to its allies that greater military investments are needed to maintain security in Europe.
Quoting RAND again, keep in mind that deployment cost are only a fraction of overall defense costs (you still pay gear, salaries and housing, though you forego local basing benefits to your own economy).
The cost of keeping U.S. forces in Europe must also be matched against the benefits that the United States might recoup from reducing its troop count. Even advocates of reduction agree that the concrete costs of the United States' European presence are small—on the order of hundreds of millions, not billions—relative to the overall defense budget. Further, that the primarily light infantry and armored formations present in Europe would probably not add as much value in the primarily naval and air contingencies imagined in a conflict with China.
Not sure about Europe but South Korea foots a considerable part of the bill for the US troops there (and see next paragraph about ICBMs).
Another factor is that ICBM interception technology is often strongly geared toward "boost phase intercept", nearer adversarial launch points. That was the rationale for bases in Poland.