Any military intervention into North Korea would have to be a joint effort by the USA and China. Neither power would tolerate the other moving troops into North Korea unilaterally.

Secondly, even a successful intervention would be a humanitarian crisis that would require a decades-long commitment by the powers involved.

And finally, North Korea has nukes. Any intervention would start off in a mad scramble to destroy, capture, or neutralize North Korea launch sites, and then to secure any surviving warheads.

This also partly explains why other countries are willing to send aid. Nobody wins if North Korea collapses.