Because we never know where the next threat will come from, or perhaps the threat after that.

Your country may be perfectly at peace with the US now, but there is no way to guarantee that peace unless we have people to continually watch for potential threats. Even that is, in itself, no guarantee, but it's better than nothing.

Maybe someday, mankind will be able to share universal goodwill and peace, but until that time, trust, but verify, at a minimum.