The states can assign them proportionally, but why would they? Again, when we say "the state", this really means state legislature. And, generally speaking, control of state legislature correlates pretty strongly with popular presidential vote in that state. So why would the party that controls the state, and which currently gets all of its electoral votes, effectively volunteer to surrender some of them to their political enemies?

Now, if one party generally expects to win the national popular vote more often than not, it might sign up for such a scheme on the condition that other states do - this is what the National Popular Vote Interstate Compact is. But, by the same token, it means that the other party would generally expect to lose the national popular vote, and thus would do the best it can to block such a proposal from going forward by preventing states under its control from joining (or by other means; e.g. North Dakota actually passed a law to block release of its popular vote data until after EC vote specifically to disrupt NPVIC).

Thing is, nobody in position of power today actually cares what the "real intention" of EC is or ever was. The only thing that matters today is winning elections, and so a bipartisan agreement on any electoral reform that upsets the existing balance of power is extremely unlikely.