Those sculptures look kitschy and childish when painted.
I assume they went for that style of painting because it was historically authentic? Presumably they don't want to decorate a 50 BC statue with 1750 AD painting techniques.
We should also note that many of these statues were placed outdoors. Colors might seem bright and garish in gallery lighting against muted white backgrounds but wouldn't pop quite so much under bright sunlight in the middle of a public square, especially once they develop a patina of dust and the sun starts bleaching them out (which would take no time at all under the Greek sun).
This makes a lot of sense, the artists who made the statue were likely more able at the coloring aspect than simply solid colors