To expand Jess Austin's point:

The ability to focus on a few swing jurisdictions also helps make voting itself (before any recounts) more observable and fraud-resistant.

Consider the alternative, with a vote anywhere worth just as much, be it in the most deep-red Southern suburbs or dark-blue urban fiefdoms. In a close election, a few thousand votes *anywhere* could be the swing. The overzealous and marginally corrupt in every partisan stronghold would have both motive and opportunity to 'run up the vote', knowing there's not enough scrutiny to reach everywhere at once (and confident that their local dominance could cover up any tricks).

Reducing the contest to a few swing jurisdictions lets every part of the process concentrate: the advertising, the GOTV, the ballot/count monitoring, the post-election analysis (and if necessary recounts). A more dispersed process could easily have more fraud and more randomness in its final result, which hardly seems beneficial for democracy.

(I'm happy with the swing states being the 'jury' which decides the presidency for us. I would even be happy explicitly delegating the choice to a few hundred or few thousand voters. They'd be chosen in advance by lottery, intensively campaigned by the candidates, suitably compensated for their efforts, then given time to deliberate. That'd be a much easier process to monitor for fraud and corruption than a natiional popular vote, and its 'error bars' no greater.)