And how would that help? How do you know that the code they published is the same as the code installed on those machines? You need voting machines that leave a paper trail (which the voter can verify while voting), that way the machines can provide quick results while the final results can be counted by hand.
We had them, for decades. They involved the use of punched cards and a butterfly ballot. The voter could physically inspect the ballot to determine which holes were punched. Sadly, the "hanging chad" of Bush v. Gore (2000) pushed many states into using electronic machines, with predictable results.