Whatever the reason is, I find it incredibly annoying, we spend considerable amounts of time creating Livery's only to have it obscured by some ugly big number that seems to serve no purpose.
This is easy to overcome, just place a numberboard as part of your livery in the same place as where the Sports mode ones appear.
Sometimes it needs a little artistic licence, but its hardly a big deal.
For example, this livery (not mine) has the Dekra number in the same place as the original livery
However, when used in a race it looks a bit odd because you end up with the Dekra one, and the Sport mode one,
This is my version with them swapped round.
So now in a Sports race mine just has one number, and when used in lobbies it shows the Dekra one.
All you need to do is have the Sports number showing, then add a square decal and resize it so it is just slightly smaller than the Sports one.
Then remove the Sports one and that is your area in the square to create your own.
This obviously allows you to not have your design spoilt by the Sports number, and not having to leave an empty space.
Three examples of my liveries where the Sports board covers my one