Why are ghosts and other spiritual beings depicted as wearing clothes?

"So down the great dragon was hurled, the original serpent, the one called Devil and Satan, who is misleading the entire inhabited earth; he was hurled down to the earth, and his angels were hurled down with him." - Revelation 12:9.

Did you notice the "misleading the entire inhabited earth" up there?

When spirit beings had to appear visibly before humans in Bible times, they usually materialized human bodies for that purpose. For example, remember the angels that helped Lot's family flee from  Sodom. A materialized human body was a fabricated body. For obvious reasons, the angels did materialize clothes along with the  bodies while visiting God's servants. - Ge 19:1-11.

In the case of Eliah and Moses, that was "a vision," a movie-like projection. (Matthew 17:9) It is reasonable to conclude that God wouldn't fabricate a projection with the characters with no clothes on.

As the Bible explains, spirit beings, including demons, are not flesh and blood. On top of that, God removed demons's ability to materialize human bodies. Therefore, if they manifest themselves "visibly" nowadays, both their bodies and clothes would be fabricated images, made-up projections; they would be fake. That's part of their "crafty acts."

For that reason, the Bible warns: "Put on the complete suit of armor from God so that you may be able to stand firm against the crafty acts of the Devil;  because we have a struggle, not against blood and flesh, but against the governments, against the authorities, against the world rulers of this darkness, against the wicked spirit forces in the heavenly places." - Ephesians 6:11-12.

Because spiritual beings are notoriously modest.

Great question.  Just one of the many many inconsistencies in any religion that touts the life after death nonsense.

Here's another one....

The bible of the Christian religion writes about the "rapture" and the reincarnation of all those who are saved and lifted into heaven during the end of days.  Are those reconstituted people clothed?  Who's their designer?  What are the clothes made of?  Are they dressed for Nordic weather or Med weather?

There are just so many things we should know.

because that's how people portray these fictional beings

Ghostbusters probably isn't the best depiction of the afterlife.

All our stories are designed to separate us from animals

All this stuff is ' faith based ' which means whatever you imagine it to be..

Because of the taboo about nakedness that is inherent in society.