This would make candidates campaign all over the country, not just nine or ten States.
As 2018 illustrated, it makes it too easy for Democrats to win. Dems had a 40-year majority in the House before 1994 which bred entitlement. Imagine if the presidency was in one party’s control for 40 years.
Plus, what does somebody do who wants the president of one party but the congressman from another party?
I tend to agree with your idea on each district ‘mattering’, but it would invite tinkering to redraw the districts. I’d like to see some law passed before we get to that point where maps have to be limited to actual county lines.