People aren't "fine" with it exactly, it's just that the alternative usually looks (potentially) much worse.
Regime change would mean war with North Korea, and at the very least that'd be a ton of artillery shells landing on Seoul and killing a ton of civilians at the outset of the war.
North Korea by itself would obviously be much weaker than a presumed SK+US force, but then you have to worry about China potentially getting drawn in, and now you have two nuclear superpowers fighting each other in all-out war. Nobody really wants that.