Starter is spinning cleanly but not engaging the flywheel?

I have a 2007 Mercury Mariner with the 2.3L engine. It won't start and you hear the starter spinning cleanly with no grinding. I swapped starters, same issue. We bench tested the old starter and it was working. Tested relays/fuses. Bypassed S control wire. Still spins with no engagement. No flywheel damage. Tested battery and it was good. It seems that the starter isn't getting the needed power to kick out the bindex to engage the flywheel and start the engine.