Probably because, in films, fantasy needs to be paired with a strong genre. The audience necessary for movies to be successful and warranted is many times the equivalent for published work. Even today, in the post-LOTR films era when fantasy arguably became a mainstream genre, it is still fighting an uphill battle to prove itself and to attract movie audiences.
One consequence of how niche fantasy as a genre is in movies is that it will usually need to be paired with another genre to attract a sufficient audience. The same thing with sci-fi. All fantasy successes have done that, the latest of which, Game of Thrones, is as much fantasy as it is a political thriller and a horror series. The Witcher is more light-hearted, but it would still be too much of a genre piece (a fantasy hardboiled thriller) to satisfy your requirements.
And just as "light-hearted vanilla" as a genre by itself is not attractive to movie audiences*, that matched with fantasy is also not attractive to audiences. Therefore it's only rarely made.
*You mentioned explicitly that you exclude products with goofiness and comedy, so something like the Jumanji reboots would not count.