Because the constitution specifies that the several States (not the people) shall elect the president.
Just make the vote of each state proportional to the number of people who voted in that state.
Or change the Constitution.
The electoral college is as much of a mess as it is in part because it already includes the proportional ratios of the House into consideration (and as pointed out in related threads, a large reason why it would ever tie in the first place, and isn't just some odd number derived from the number of states). Surely in the case of an EC tie, leaving the House to vote as the House, proportionately representative, is no less a representation of the votes of the states than the electoral college itself at that point?