Earlier quoted context omitted.
People definitely do not agree with what you’re saying. Not everyone thinks that swearing an oath on something is only meaningful if that thing has some kind of power.
To elaborate on this, if I swear on my mother's grave, I'm pinning my respect for the oath to my respect for my dead mother. If I violate the oath, I'm also violating something I consider sacred. My mother never has to find out for me to feel the consequences. Probably this doesn't work on everybody. Even if it only works on 5%, it's essentially free so why not?
The oath is a symbolic ritual that communicates something to other people around you.