Considering that the President, Chief Executive of the Federal Government, represents and is elected by the States, a nationwide popular vote election is a violation of State sovereignty.
The United States of America is exactly what it says: A Union of sovereign States. It's very sad a lot of people don't understand this because they slept through civics/history class.
The roles and capabilities of the state and federal government have evolved since the 1700s.
The state level of government feels more and more like a historic novelty carried over as "we always did it this way" and "it would require far too much political capital to touch"-- America's answer to the House of Lords or the Governor General.
State sovereignty is how and why certain States legalized marijuana while others don't, why certain States impose a given tax the way they do and others don't, why States each have a National Guard that is legally independent of the Federal military and thus from the authority of the President, and a host of other matters that are determined or reserved by a State and its electorate.
The United States of America is a very vast country literally spanning an entire continent, State sovereignty is more important now than ever before because the needs and desires of the peoples within them can and will vary wildly from each other; they varied wildly with just 13 States across the eastern seaboard, let alone 50 States spanning a continent and islands in the Pacific.
The Federal government exists to better enable functions of government where the States have a unanimous consensus, and to that end the Federal government including the Presidency exists strictly at the pleasure of the States. For all other matters where States can and will disagree, the States reserve the power to decide by themselves for themselves.
Okay. The equivalent at the single voter level is if a voter is undecided between Candidate A and Candidate B. After much contemplation this voter decides she's 50.5% in favour of B and 49.5% in favour of A, so heads to the polls to cast 100% of her single vote for B.
The 538 Electors are derived from the 435 Representatives and 100 Senators in Congress, with an additional 1 Representative and 2 Senators equivalents in Electors allocated to the District of Columbia (specifically the same count as the least populous State in the Union, currently Rhode Island with 1 Representative and 2 Senators).
That means of the 538 Electors, 436 of them are allocated according to State population counts as recorded in the United States Census taken every 10 years.
And the outcomes are frequently undemocratic. Only in America is it fair for the smaller number to be bigger than the bigger number, which just so happens to conveniently benefit the minority, who then twist themselves into knots to "well, akshually" us all to death.
In Australia the seats in the upper house of Parliament are strictly divided between the States with each receiving 12 seats regardless of their relative populations (two territories which are not proper states also receive 2 seats each). I suspect some other countries may have a similar federal model where there is some kind of barrier to stop larger states from being predatory towards smaller states. If you check wikipedia (https://en.wikipedia.org/wiki/Federalism) you can see some other countries like Argentina have a similar model to Australia. I think in Australia there was probably similar historical reasons for the Senate as the Electoral College in the USA. The colonies in Australia were self-governing and then decided to create a central government and the Senate was a way to give the smaller colonies confidence they wouldn't be taken advantage of in the future.
The United States of America is exactly what it says: A Union of sovereign States. It's very sad a lot of people don't understand this because they slept through civics/history class.