I can’t imagine why the showrunner or writers of the source materials wouldn’t prefer creating new stories if so many other showrunners and writers were allowed to. Even Game of Thrones (through S5 or so) and Outlander feel thin and predictable if the viewer has also read the original novels. A number of DC animated movies come across as comic commercials, a way of getting you to read the underlying books and then hopefully other comics as well.
I mean, sure, there are classic movies which (often loosely) adapt great novels in interesting ways, but in that case these adaptations would have to be rather innovative with a strong focus on the target medium, not just conversions for the sake of getting fans to read more tie-ins.