> The founder's obviously knew about proportional representation and they deliberately chose to disregard it. Shouldn't that give you pause to figure out why?

Turns out the reason was slavery. A huge proportion of the Southern population was enslaved, and slaves didn't vote. The free staters didn't want slaves to even count toward state population for the purposes of congressional apportionment, while the slave staters did, so they compromised on counting the slave population at 3/5 of the free population. As a result, prior to the abolition of slavery, the votes of free men in slave states were always represented at a higher proportion than the votes of free men in free states.

Many of the founders were wise and educated men who thought carefully and critically about how to establish a sustainable republic. But to do so, they had to win the votes of a cross-section of the American establishment of the time, many of whom thought that owning human beings, using them as farm labor, and whipping them if they disobeyed were morally acceptable things to do. And upon making a series of political compromises with these people, they had to rationalize those compromises in such a way to make them palatable to the general public.

Most of them were perfectly fine with owning people.

A lot them even owned their own slaves.

Indeed. With our current system, rural voices are at least marginally relevant. With a purely proportional system, rural voices would not even be heard.

The major reason is that smaller states wouldn't have joined any proposed Union that made them irrelevant, so the founders had to have things like the Senate to get them to join at all.

This is, of course, no longer relevant.

If it’s no longer relevant, it should be possible to convince small states both red & blue to accept an obviously superior system.

But it is still relevant for just about all the reasons it was relevant then.

I think if you had some way of forcing the issue, and telling states like Kansas and Mississippi that they didn't get to have disproportional control over the federal government anymore, and if they didn't like it, they could just stop being subsidized by California and New York, either they would go along with it, or they would secede from the union and deteriorate into third world countries.

Why would states persist in a union that made them irrelevant? The politics are still as relevant today as it was then.

It’s way harder to leave than it was to refrain from joining.

Another major factor is that Americans mostly see themselves as citizens of the USA first, and citizens of their state a distant second.

Because the alternative is worse.

They also didn't choose to tie electors to the per state popular vote and chose the electoral college in part to prevent the election of terrible candidates.