"Saluton" finiĝas per "n" sekve saluton estas rekta objekto, sed la rekta objekto de kio?
"Saluton" ends with an "n" thus it's a direct object, but the direct object of what?
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Saluton is simply a shorthand way of saying the complete sentence:
Mi donas al vi saluton!
The reason for the accusative ending is to denote that the greeting is the object, of which I am giving to you.
You'll see the same pattern with other things, such as
PMEG has further explanation if you want to read more about it (in Esperanto).