Personally I think it's because the game doesn't have a tonne of depth to its mechanics, so it's hard to diversify yourself mechanically. The game over emphasises the Rock Paper Scissors style design where too many heroes just beat x hero, because of their kit, because of their character.
So with a low mechanical skill ceiling, it leaves very little room for the players themselves to have their individual characteristics influence how the game is played, and instead, it comes down to which characters are picked, and the overall team strategy that's employed.
Paladins and typical mobas have less of these balance issues because while they have overpowered characters and balance issues, they have tremendous skill ceilings which means that the meta is perpetually shifting and that the characters you pick are not the be and end all of a match. In games like Smite, balance issues exist but these are much more malleable over time, without influence of the developers. You see people finding out new and creative ways to play different characters because of how diverse the game is, and how high its skill ceiling can be, which tends to prevent scenarios where everyone is playing the same set of characters.
Essentially, I'm saying that it's not about the game being unbalanced, that's not the issue, these games are inherently unbalanced because of their diverse roster. However, the issue in Overwatch is that the puzzle pieces here are too simplistic that players figure out that meta too quickly, and the skill ceilings among most of the cast too small to allow that meta to fluctuate dependent on player ability. As such the problem here is that you get these waves of stagnation, as players figure out the meta, play it to death, and then hate the game.
The issue isn't the character specific changes that Blizzard make, it's instead at the heart of Overwatch's design.
