In both Ancient Greece and Rome slaves were kinda expensive. It made more sense to hire free men who would expect a share of some bounty and improvements in their social perception, if they survived.

Also the rows were not the same as in Venetian galleys, one oar per man against one huge oar per few slaves, losses among rowers purely due to collision of ships were smaller, mobility of rowers to fight when ships got close was bigger. Venetian rowers had to be trained like machines and worked to death like machines, for the oars to move at all. But the resulting efficiency was bigger.

Purely economical reasons.