Why am I charged for monthly apple music subscription

If you want to cancel, see instructions in See or cancel your subscriptions - Apple Support

Because you must have subscribed to it at some point. Subscriptions do not magically appear. You would have needed to provide your passcode, touch Id or face id, and agree to it at some point.

You may have thought you were buying a song or album, but were actually subscribing to Apple Music by mistake.

As said it can be cancelled at any time if you no longer want it.