To do that with in person voting you have to physically move around enough people with enough fake IDs to actually swing the election. The difficulty of doing that silently without anyone saying anything is a protection in and of itself. On top of that you have to get that many people on the voter rolls to start with.

that's entirely a question of scale - I can imagine that it'll be trivial to scale 'online' voting fraud if you have credential access, whereas doing it in person naturally limits the total number of times you can do it.

Committing identity fraud in person, in locations likely to have an elevated police presence, is extremely risky and getting caught carries severe consequences.

More fun Cook County details: not only is there not an elevated police presence at our polling places, but the election judges are charged with making sure the local police are not hanging around within like a quarter city block, since the local police don't have any authority with respect to voting. We got to chase a cop away at our place.

(The Cook County Sheriffs police our vote, but there aren't that many of them, so if they're going to show up, it's because a judge called them out.)

Every poll site has a poll book of eligible voters (for that location). Voters present their ID, sign next to their name, are issued a ballot. In order for mass fraudulent voting, impersonators would have to choose IDs (of persons) they were very confident were not going to vote.