Sending sacks of votes from Maine or Carolina to Philadelphia or Washington wasn't conceivable. Some of those attending the Constitutional Convention had been waylaid themselves on the way to Philadelphia. But Madison and others didn't want the alternative, a parliamentary setup. That's where Congress selects the President. They decided Electors could be from congressional districts, but as ordinary folk, they'd be apolitical.
They failed to predict the power of political parties.
Votes were collected within districts and Electors representing them met in state capitals, then together went to the nation's capital. Parties quickly realized they could dominate a state with a "winner take all" rule.
In every state, a law was passed that all Electors had to vote for the state's winner, regardless of a locality's intent. The dominant party in a state maximized their influence. If their chosen candidates did not win at the national level, the whole state was in opposition, despite voters in the state who weren't.
Because of the 3/5 compromise, slave states got more Congressional representation per actual voter. The Electoral College baked in this advantage, since the number of Electors was based on Congressional and Senate representatives. As slavery became increasingly condemned, slave state Electors were a solid block of opposition to reform. Any southern voters who opposed slavery were silenced, which promoted secession.
By 1840 Electors were an anachronism in many places, because of railroads and telegraph, manufactured paper and inks. After the Civil War there was no excuse for Electors. They preserve party power at the expense of the state's disenfranchised voters.
It's been possible to have elections by popular vote for 150 years.
21st century technologies transmit vote results in seconds and minutes, not weeks and months. Monitoring is continuous, redundant, and necessary.
Electors have no cause to exist.