Bases overseas are typically part of "Security Guarantees" which are strategic to stop our allies and strategic interests abroad from being aggressed against (think north korea vs south). You could say "fuck south korea" but they supply alot of critical technology to the USA. Curbing Chinese expansion and aggression in the pacific is important too. There's more of course. Having bases let us project power wherever we need it in the world in a very short time. The military is quite large, you'd be surprised how much support staff you need in the brass to keep the lights on with how complex the armed forces are now (tech integration is huge) and things are much more complex than they were in WW2.

Now do I agree with us consistently being world police? No. That's probably why Trump is so adamant that NATO members get GDP defense spending (1.5 percent to infrastructure!) up to 5% vs the 2% goal of his first term. It forces member nations to adopt our "peace through strength" doctrine if Trump won't honor article 5 invocations if members aren't paying their fair share, which in theory could reduce the amount we need to play world police.