Early writing wasn't as standardised as it is now, and most reading would be done aloud in more or less formal circumstances, so the maximum efficiency we strive for today was probably not a consideration.

And since writing on stone is a pretty cumbersome activity it was probably natural to continue writing next to where you just finished a symbol.

Note that cuneiform (which of course predates Greek by a wide margin), written on clay with a reed, was not written boustrophedon.

On the other hand, Egyptian hieroglyphics, though carved in stone, weren't either - instead they were very flexible on writing direction.

I suppose my only real point is that the Ancients didn't see a great need for a fixed writing direction.