I regard fanfic as a niche form of writing - more of a shared hobby than a fiction genre. In some cases the writing is quite good, in most cases it's fairly mediocre, and in many cases it's truly terrible. It *may* lead to more original writing, but most people who write fanfic stay in that mode.
It's not that it is not a creative endeavour, it's that the creativity required is more limited than that required by original fiction. The author has to write the plot and descriptions, and has the wording to worry about, but they don't need to develop character and their interactions, and back story, do world building and all of those things. It's even more pronounced in genres like science fiction and fantasy where world building is a major part of the writing.
In some ways it's like the difference between doing a cover of a song, and writing your own. Interpreting and playing someone else's music takes hard work, creativity and skill, but no-one is going to claim that it's the same creative process as writing original music.
Accessibility is a point as well. In general, fan fiction is only accessible (or of interest) to people who are familiar with the work being used, and is only read by other fans of the series. Crossovers are even worse, as you have to be familiar with two separate works of fiction. In most cases, they *can't* be successfully read independently, which ties into the more limited type of creativity part. Fan fic also cannot, in general, be published.