Why do congressional districts have to be confined within state borders?

[AGA]

Another way to phrase this question is: why can't congressional districts transcend state borders? I understand that the House of Representatives votes as state delegations when there is no electoral college majority in the presidential election, but this could just be replaced with a normal vote.

If congressional districts can be in multiple states, it would be easier to have population equality. For example, in Montana, there is an at-large congressional district that has over one million people, much more than the average district population of about 710,000. Montana could have a congressional district with around 710,000 people in it, and another one that has the rest of the states population and has some area in another state as well to reach population equality. The issue of population equality also exists with the states of Wyoming and Vermont, which each have around 600,000 people.