Because now that this is a 3D game that'll take a lot more effort to make, the dev wants to earn money off it. Issue is, you can't really make money off copyrighted characters unless you want a DMCA from Sega.
Realistically they don't *need* to commission 3D models or even make new custom ones as there are plenty free one's that already exist.
Granted it's understandable for a dev to want a set unique style for their game. And given mobile limitations there's both a texture red & poly count max regarding model quality.
Imo it'd be better to just give users more parts to work with on a Base default model. Different tails, ears & adding horns would create a significant amount of potential combinations.
If you don't have specific characters & simply have lots of parts that allow users to create the characters they want, you can't really get DMCA for that.