They serve a ministerial function. They have no authority to inquire into the age, citizenship, or residency requirements specified by Art. II. They vote. Period.
Maybe you're familiar with the Federalist Papers written by James Madison, Alexander Hamilton and John Jay. Federalist Number 68 concerns "The Mode of Selecting the President."
Federalist No. 68 explains why such an important function (choosing a President) was delegated to a small number of very important persons (electors):
"It was equally desirable, that the immediate election should be made by men (the electors) most capable of analyzing the qualities adapted to the station, and acting under circumstances favorable to deliberation, and to a judicious combination of all the reasons and inducements which were proper to govern their choice. A small number of persons, selected by their fellow-citizens from the general mass, will be most likely to possess the information and discernment requisite to such complicated investigations."
Obviously, the Founding Fathers thought that choosing a president required "complicated investigations" and that the selection of a president should be made by people "most capable of analyzing the qualities adapted to the station, and acting under circumstances favorable to deliberation, and to a judicious combination of all the reasons and inducements which were proper to govern their choice."
The Founding Fathers saw the job of an elector as being much more than the perfunctory, ministerial function that you envision for them.
It has to be conceded that a "moral certainty" level of confidence does still allow for some small possibility of error, but no system is 100% perfect. We shouldn't abandon a system just because we may be unhappy (for other reasons) with the selection of a particular president.