This would only be true if all you did was stand still and click your attack button. Oh, and if there were no choices between abilities and runes because they all did the exact same amount of damage with no other effects.
If you take into account how the game is actually played, the immense variety of options, and probably most important, game mechanics that are completely different for every build, then you see that it is not, in fact, simple at all.
The reality is the developers have no idea, and no way to have an idea except for one: to look at the data from the players. At that point, if they want to “balance” something, they can make a change to a build, either in mechanics or numbers, but almost inevitably, the players will do something unexpected: like use a different rune, or change which jewelry they use, or decide it’s worth theory crafting the best use of a totally different build, and viola, the data has changed. Time to rebalance.
You’re also ignoring an important facet of the game: if all builds were identical, then the game would be dull as mud. A large portion of the enthusiasm for playing the game, especially in seasons, is to try out the most exciting builds of the moment. Take that away, and the player base would quickly dissipate. Having better builds is better for the game.
Oh, and don’t forget about player skill. For example, Earthquake Barb isn’t considered a top build, but this Rage guy is apparently amazing with it. Or Frozen Orb DMO, a low power build, except in that one guy TinneMone (I’m sure that’s spelled wrong and I don’t know if it’s a guy) hands. Dieoxide makes Marauders look much better than it is on average. And so on. There’s no way to balance around this, other than to ignore them, which would defeat the whole purpose.