@Big_Boss
Someone a while back once told me they do it to keep up sales. People relate better to a jack of all trades person who has a decent ability in everything, because something they personally are good at, will overlap. A character who specializes in a certain area, wont be as relatable to many of the readers.
Also, even if the "best girl" is hinted towards, keeping her "average" makes the potential for the other female characters seem a bit higher. It also allows for more specialized 1 on 1 time with all of them (like a "date" or a random "coincidental encounter" flashback) that would not be considered NTR. For if they ever needed a filler chapter that is.
Basically, if this were compared to fishing. The author would be using a large net instead of a specific bait proven to be effective only towards a certain type of fish.