Yeah, but it's also not as easy as you make it sound here. Not everyone, I'm sure even among top players, has the ability to focus their style towards more than a single character. There's also the matter of time, where you have to think about whether it's best to spend the time to specialize with a single character, or possibly be less efficient with that single character with the benefit of having another character be at their level.
It's especially hard when it comes to "character specialist" type players to dedicate time to a secondary, or even have the will to do so. In most cases, you won't see Dhalsim or Zangief mains play a second character, for example.