Americans support a National Popular Vote for President primarily because they agree with the principle that every vote should be equal (1-person, 1-vote), and the candidate with the most votes should be guaranteed to win.
Looking deeper into the current system though, it's clear there is more than just fairness at stake. In 2016, the current system put the candidate trailing by 3 million votes into office, eroding confidence in our election process.
In 2020, the popular vote winner prevailed under the Electoral College but the system nearly backfired. If 23,000 voters total across three states had changed their mind, the 2nd place candidate would be in the White House despite trailing by more than 7 million votes.