Because as soon as you get a receipt you can be coerced into voting a certain way and be forced to prove it with your receipt.

Absentee ballots are kept with their envelopes until they're ready to be counted, so they can be thrown out if need be. Polling placed mix all ballots together so there's no going back.

The optical scan machine is simply a convenience. Your registrar should be doing a manual count of a random sampling of the ballots, and any close race should trigger an automatic hand recount. On top of that, any candidate can chose to pay for any hand recounts they want of the actual ballots, not the optical scan machines memory card.