Because while you’re living in peace, the other guy will come for you in the night. It’s unfortunate but that’s how it’s been throughout human history. The only societies which have enjoyed peace are those that have the overwhelming means to destroy their enemies.
China has an ICBM arsenal that can hit the US and is developing a healthy conventional military as well. Therefore, given that they have overwhelming means to destroy us, as long as our leadership doesn't go completely crazy we won't try to come for them in the night and will be able to continue to live in peace.
