Essentially what you're saying is that you want the content to be more easily accessible for people who didn't spend time leveling in eureka for one reason or another. Whereas Yoshida has stated on multiple occasions that Eureka was designed to be content for old school MMO players who enjoy long grinds and grueling endgame content. In this very interview he says the public dungeon is not "casual" content, and casual players should avoid it.

So to answer your question, why is there an alternate leveling system ? Because it was designed that way to be a long grind on purpose. The public dungeon is content for the type of people who enjoy these kinds of grinds. If you don't like it, it just means you're not one of the people this content was intended for and you don't have to do it.