What’s with the holy snake oil?

Evangelical Christians and snake oil go together like 3&4 but why is this?

My mother would spend money buying oil, and getting our pastor to bless it in hopes it would heal her arthritis (it never did, surprise!) but I’ve always wondered how did this absurdity start.

Is there something in the bible that states normal objects can be transformed by humans & given powers?

A somewhat related side question, how did “fighting against principalities” come to mean fighting against demons and the occult rather than terrible people and their ideologies?

Thank you for any input in advance? X