Because you have to be physically present in order to do that, and nobody wants to risk getting thrown in jail if they pick a name that’s already voted, or that the poll worker happens to know.
However, the article talks about "election systems" which encompasses much more than just voting machines. Voter registration records are a prime target and those are usually (always?) connected to the internet. You don't need to actually alter any votes to swing an election. Just delete or alter the registrations of some people who you know will tend to vote the other way.