In other words, although voters might be under the impression that they get to choose the president, in fact, the Constitution does not mandate that role. Ordinary voters have a say in the process only as a matter of legislative grace within each state, not as a consequence of constitutional protection.
In an age of political hardball and rampant political gerrymandering, there are at least three ways in which partisan state legislatures wielding this power could create difficulties that call into question the fairness of the election.