It's a question of priorities. It's easy to get into a war thinking it'll be short and easy then once you're in it's poisonous politically to not fund the war because it gets spun as not supporting the troops. Then it's hard to pull out of place like Iraq even if you don't want to keep funding the war because now it'll just become an broiling pot of lawless extremism because it's hard to rebuild a state and impose outside political systems at the same time.

Also a lot of the larger bases like those in Japan and South Korea are partially paid for by the countries there as part of getting US troops for defense.