My opinion is that there are two major reasons.

(1) Volunteers have other commitments. This could be their "day jobs," families, etc., but generally unpaid projects are "passion projects" that aren't necessarily allocated the same time as other commitments that the volunteers may have.

(2) It's (generally) not possible for a fan made project to have the same quality of work as the original off of which they're based. Things like high profile games have experienced, well compensated developers that spend years at work to complete -- with huge teams. Hobbyists have neither that experience nor that luxury.

And lastly, yeah - legal issues.

I guess fanmade projects are typically run by hobbyists, rather than professionals. Professionals would know better not to start one with an IP they don't have the rights for.

So, hobbyists, lack of knowledge and experience to run an actual project, and only fueled by their passion to recreate their memories, decided to start one. Those who failed, we never heard of them. Those who succeeded got the letter from lawyers.

fanmade projects are often started by young unexperienced enthusiasts.
-> The projects take very long time.
-> People growing up mostly develop different interests with time.
-> The project dies.