As tradition, the developers see themselves as gods on earth who cannot be given any sort of feedback without hurting their ego.
Many other open source tools are beloved and used despite their flaws. Gimp is not built for anyone who isn't truly invested in it, making it a niche piece of software.
And the direction of 20 years of a software can tell what kind of contributions would be acceptable for people. I don't think anyone coming in a redoing the whole UI would well accepted, given how little care is given to the same old feedback.
Many other open source tools are beloved and used despite their flaws. Gimp is not built for anyone who isn't truly invested in it, making it a niche piece of software.
And the direction of 20 years of a software can tell what kind of contributions would be acceptable for people. I don't think anyone coming in a redoing the whole UI would well accepted, given how little care is given to the same old feedback.