Because sidekick rules are not grounded into simulationism nor world consistency. Just like you can't by the book have a PC that's exactly the same as one of the NPCs in the MM appendix.
We aren't talking simulationism though, we are just talking about relationships of power. Sidekicks are AWFULLY powerful compared to the normal PC classes in many ways.
In theory, from what I can see, they relieve the notion that you are limited to a maximum of 20 in a statistic for example.
The bonus of a +2 to hit, and the ability to continue to add to their ASI's are also boons that keep them pretty relevant.
I'm starting to see ways to abuse them in a min/max way perhaps, that if played right could outstrip PC's in general.
The rules are VERY vague on them...which is part of the problem and perhaps part of the advantage if one is looking at loop holes.
For example, something to wonder about...can I multiclass my sidekick...what if the sidekick is being used as a PC class?
If so, this opens up an entire separate avenue of abuse of these rules in combination with other classes, especially when looking at synergistic lines of powergaming.