How to explain Dispel Magic?

One of my players asked me, if they could use Dispel Magic on a seemingly magical creature (Skeleton). RAW I would only allow it, if the creature was a sustained summon, but I am at a loss how to explain it logically.

In this specific case I could argue that the Skeleton has been created by magic, but is now fueled via Void energy, which is not(?) magic.

How would you explain that Dispel Magic doesn't work on creatures partially made up of magic?

Thank you.