Why is being naked considered dirty?

Because god said.

god did not say naked is dirty

when Adam and eve ate the forbidden fruit, they then saw themselves different and were embarrased

also naked is only considered dirty in public

and that's by the prudes

noone thinks a naked baby is dirty

nor does anyone remain clothed while in a shower or bath

A4. Nakedness is appropriate when by oneself, or with someone similarly inclined. And there are nude beaches in some places, and nudist societies are common round the world.

In overcrowded, undereducated societies, nudity is frowned on.