Remember that US troops were stationed in Berlin/ West Germany prior to German unification (i'll admit its not the exact same situation since the East German economy was relatively much stronger pre-unification than the current DPRK economy).
If you're suggesting that we should just let the DPRK take over ROK... well, I don't think any of the ROK inhabitants would like that.
The fact that the American military gets to decide what the Korean people want or what the reunification process will look like, is the single biggest factor hampering the peace process.
Now, with that being said is American military presence an issue for North Korea? Yea definitely. But if the U.S. withdrew troops it's not like Korea would all of a sudden be reunited. There's a laundry list of issues that would need to be resolved. American military presence is just North Korea's go-to excuse because they seek to divide the South Korean and American alliance.
If North Korea wanted to, they could just open the borders, stop starving their people, and all that jazz. Why do they need to do all of that crap? Because 25,000 Americans are in South Korea? Yea ok.
There are actually evil people in the world that don't live in America. Many of them live in North Korea, happily starving people, raping women, and executing people for no good reason.
It's not that simple. American presence on the Korean peninsula makes reunification efforts overly complicated. By some estimates, reunification would grow the Korean economy to be the third largest in the world. It would also give Koreans access to Russian pipelines and a large border with China. When climate change opens up arctic trade routes and frozen arctic resources, Russia will be in the position to benefit the most (although Putin relies too heavily on his tsarist-Stalinist strain of conservatism to be able to exploit the economy as efficiently as liberals or perhaps Chinese communists might). As convenient as it is to have a powerful ally in the U.S., the U.S. is no longer in a position where it can dictate the behaviors of foreign nations as it once used to, especially regarding China. That's simply a reality that Koreans have had to come to terms with, despite Americans seeming to believe that things can just return to how they used to be half a century ago.