Why are the WWE still making John Cena out to be Eminem?

It's because the John Cena character was modeled after Eminem who Cena was a fan of. The WWE originally intended him to appeal to the middle class white kids who liked rap demographic which was very big circa 2003.

Its the same demographic Mathers himself capitalized on to become rich all be it unintentionally. When Eminem sided with the likes of Dr Dre, it gave him credibility in the eyes of some black people. Couple that with the fact he is white gave him mass appeal.
Other rap artists have pointed this out in their music.

If Eminem/Slim Shady/Marshal Mathers didn't exist the John Cena gimmick wouldn't have worked and Cena probably would have been seen as a joke.

Eminem actually somewhat de-stigmatized the white rapper idea by making it cool and that helped in a big way toward John Cena himself gaining popularity and be accepted by fans black and white.

Cena is virtually a wrestler version of Eminem. Once he got popular by being Eminem in wrestling, Cena then turned to the make a wish foundation superhero gimmick and got all the kids on side but he got a huge head start because of his similarities to Eminem.
Like it or not.