The scan groups which have their own sites, are paying themselves for maintenance of the site. Also they are buying raws. They just ask for donations, and to read the works of their site, nothing more.
Mangadex don't need to upload the works of the scan group which have their own site, IT STEALS reader views.
In case of licensed mangas, as in Jaimini Box's issue with One Piece, Black Clover examples; actually the translation of licensed works (Viz, Kodansha, Lezhin) are bad. They skip or alter some sentences, or make ridiculous names, example with One Piece's character Roronoa Zoro (known in VIZ as Zolo...), inept centralisation of bubble sentence, lack of hyphenation, weird pick of fonts.
Also, some scan group, which TL licensed works, put the "buy from official, so to support author" on their credit page in chapter release.