Why are we guaranteed that the best hand will win, only when there is an all-in? It has nothing to do with your statement about poker being a game of skill, it's about fairness and making sure that the pot goes to the best hand at the showdown.
There are arguments on all sides of this issue, and numerous "rights" involved. It's a matter of striking a balance.
As to the first issue: An all-in affects every player in the tournament, not just those involved in the showdown.... there's a "tournament life" on the line. This is a situation where "the right to make a mistake in reading your hand" is trumped by 1) the importance of making sure every hand that could knock that player out is exposed 2) that if the player's hand will win and keep them in the tourney, it be exposed and 3) that there be no chip-dumping if a player would otherwise be knocked out.
But other than in all-ins, the TDA has recognized that A) reading your hand correctly is part of the game AND B) in non all-in situations players can muck their hands without tabling them.
This is the balance that the TDA membership has struck between these competing interests... and it's held up since at least 2007 now.