I figured it was something to do with sand being light enough to be carried by wind, but that wouldn’t explain why the rest of the world lacks sand.
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Part of your trouble is based on your mental picture of a desert being a little off. Deserts are way more complex and varied than you think (like, some are permanently icy cold!) The main feature is that it hardly rains at all.
Hot sandy deserts are sandy because there are fewer plants to live and die there, probably because it’s so hot and windy and barely rains so nothing can live. (But if you study deserts, you will find there’s actually a ton of life, just not the kinds you typically think of.)
This type of desert seems to you like they have no dirt because there isn’t enough decayed materials mixed in for you to say “that’s dirt.”
Dirt has enough rotten plant material in there for you to say “that’s not sand.” But dirt has teeny rocks in it. And that’s sand.
In places with dirt, there’s more life going on.
At the beach, the moving water makes it hard for lots of plants to live and die, and it keeps moving it all around. So we have sand there too instead of dirt.
Some moving water instead has mud (wet dirt) because living things ARE dying over many years there. If nothing died anywhere near a river for a long time, it would slowly get to be more sandy and less muddy.
That’s it really. If you removed all the life and rot from the planet, and crushed the whole planet up small, it would all be sand.
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