Plato(n) and similar masculine names
Because Latin.
When the Ancient Greek names Πλάτων, Πλούτων and others were borrowed into Latin, they were changed into Plato, Pluto. I don't know if there's a single, well-known reason for this, but Latin had a lot of existing third-declension nouns ending in -o, -onis, and these Greek names were easiest to slot into this paradigm. These names then entered English by way of Latin, where they keep the peculiarity of losing the final -n except in derived forms.