One, the electoral college itself is tied to the political power of many low population states, so any serious adjustment to this process is a pretty dangerous subject for some states, making any country wide changes very hard to start.
Two, the shift to direct election of the president (minus the electoral college) was not a planned change. If one day in the 19th century everyone decided to change, then scrapping the EC would've made sense. Instead it has been a slow process happening over at least a century to arrive at our modern system, hence the presence of vestigial artifacts like the electors themselves.
Three, the process of how electors are selected is delegated to the states, which is part of why it took so long. So for example Pennsylvania and Maryland went to a system by which one party won the entire state at once in 1789, while it took South Carolina until 1860 to abandon per district results. Maine never adopted the winner takes all approach, and assigns two votes by district and two by the popular vote tally.
the Constitution specifies that States (not The People or citizens or voters) shall choose their electors.
As you said, allocation of electors by states has been played with in different ways based on different election/ voting methods, but there's actually no constitutional requirement for States to hold a general election at all.
It's entirely up to the state legislatures, who have all since delagated the responsibility to a statewide vote.
From Article II:
"Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors..."
That's it- the rest covers how many electors each state gets.
It's questionable as to what theoretical limits the modern SCOTUS might place on this power to delegate, but they've already said that voting must adhere to "one person, one vote" principles, and have hinted that the states can't delegate the power externally (from the state). But they've never explicitly "locked-in" the requirement that any state hold a general election at all.
I think this is a bit like saying that the UK has no constitution because it’s not written down. It’s technically true, but it comes nowhere close to the actual lived experience of the people in that jurisdiction, who absolutely believe they live in a constitutional society.