Why are presidential elections so close to 50–50?

Because of the Median Voter Theorem:

https://en.wikipedia.org/wiki/Median_voter_theorem https://en.wikipedia.org/wiki/Duverger%27s_law

It's not as one-dimensional as the theorem says, but it works out the same way. Parties are collections of diverse interests that agree to work together. The interests are multidimensional but the results are not: you either win or you lose.

If you lose, you change your party (or coalition of parties) to appeal more widely. (Or don't, and continue to lose.) If you win, you want your coalition to be as small as possible over 50%, because the more people you add, the more conflicts you have to resolve.

Real life is never as simple as the axioms that lead to the theorems, but they enough to make it unsurprising that most large-scale elections are close. (The smaller the scale, the more likely some other assumptions will dominate.)

By the way, Peru's new prime minister is in dispute right now, and the results are very close to 50-50