Starter won't engage the flywheel

Driving home my car just died. Try restarting it, starter motor just spins. No grinding,no clicking, just spinning freely, but wouldn’t engage flywheel. Hot another starter figuring this one was shot, preventing it from engaging the flywheel, and in the meantime, 3 days, the battery dies completely. Put the other starter on, and did the same thing. Was told battery probably just wasn’t strong enough to power starter enough. Charging battery now. Any other reasons why it just wouldn’t engage flywheel? It’s not grinding it, so that means it’s not engageing it at all, right??