It is an entirely artificial limit imposed by the interface of the engine editor, for no reason whatsoever (RPGM has a LOT of those). Just go to the skills.json file and open it in any raw text editor like notepad, notepad++ etc. and add extra entries as many as you like and you are good to go(ofc make sure to keep the syntax intact). And as long as your don't touch that stupid "Chance Maximum..." button in the interface it will be more than happy to let you edit those skills in editor still too afterwards
Thanks a lot for the reply. I heard of that solution as well but people also said its against RPG Makers terms of service so I decided against this approach.
And yes I agree, RPG maker has some really stupid artificial limitations!
Good news is that I already came across a solution and redid the entire skill base. It currently is being tested by me and a few others. So the new version is planned to come out sometime next month^^