Why is an open casket funeral more expensive than a closed casket?

My dad died last week after a long illness. When we made the arrangements my mom wanted an opened casket in spite of my brothers and I objecting to it (its just gross). My mom insisted and said she was willing to pay the extra to have the casket open. I really don't want to see my dad's dead body but my question is why would having an open casket cost more? Seems dumb to me

Why is an open casket funeral more expensive than a closed casket?
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