States don’t get to hold elections for statewide officials (e.g. electors) where some votes are effectively discarded because of some fact external to that state election. Adding up votes from other states isn’t materially different in that regard than performing an augury.
For the same reason, the Western states couldn’t engage in a pact to elect their governors by party slate (especially without triggering the interstate compacts clause).
And why not? The constitution says that the states can choose their electors for President any way they see fit. They can be named directly in the law if the legislature wanted them to: "Our Electors are Joe Bloggs, James Jameson, and Person McPersonface". There is no constitutional limit on how a state can choose its electors.
When states started switching from the original plan of "electoral districts" to "winner-take-all", Hamilton and Madison decried it as contrary to the spirit of the Constitution, but recognized that they couldn't do anything about it because the text of the constitution says "in such Manner as the Legislature thereof may direct", and Hamilton's amendment to strike that clause and replace it with an explicit by-district electoral process failed.
For the same reason, the Western states couldn’t engage in a pact to elect their governors by party slate (especially without triggering the interstate compacts clause).