My uniformed guess. Blender was created as a an internal product by a company using it. While not very successful, it had marked oriented approach from a start.
GIMP was always a toy for programmers, without any vision behind for how to actually make it a product that commercial users would want. It’s a big job to move a ship like that.
Not a big company, but there is the Zemarmot project. One of the project members implement things in GIMP, which are needed for Zemarmot. So there are people using GIMP professionally and extending it accroding to their needs.
GIMP was always a toy for programmers, without any vision behind for how to actually make it a product that commercial users would want. It’s a big job to move a ship like that.