As someone else points out, you don't need to hack the whole country, just some key locations. Also, you could hack the source; many firewalls from a major vendor had (have?) security holes because someone checked the necessary code into their revision control system. How secure is Diebold or whoever makes the machines?

> The reported vote count is usually very close to the predicted vote count

Suprise results, where the outcome doesn't match the polls, are not that rare. Recently Sanders' win in Michigan is an example. In the UK, in the last parliamentary election, I believe the conservatives did much better than polls predicted.

> always very close to exit polling results

If true, this is a useful check.

> hacking could not be used to create a very close result that could trigger a recount, or it would need to be combined with actual ballot theft

That assumes there are ballots to steal; many voting machines are electronic.