How does that help? The paper ballots still need to be counted as they are the authoritative source of the final tally. An electronic tally does nothing useful and adds complexity.

Speed. One of the reasons some politicians want electronic voting is to have the tentative results in as fast as possible, so they can sod off to their respective celebration/mourning shindigs and call it a night. That the actual, lawful count follows in the course of hours or even days is, to them, then acceptable.

Having these machines do a preliminary tally gives you a more accurate forecast of the votes cast than exit polls.

Optical scan machines solve that issue quite nicely and more robustly, especially when a ballot needs to be spoiled due to error (it can be physically destoryed before tally instead of needing to support deletion or modification of records in the voting machine).