Simply put, a FLOSS alternative to professional software isn't an alternative if it's unable to be used professionally. The barrier for freelance and hobbyist work isn't people's concerns with dethroning these proprietary applications.
Can you get hired at a large studio with Blender on your resume? Absolutely. And Krita is slowly heading down a similar path.
GIMP, on the other hand, is dead-set on being relegated to a corner for FLOSS purists. It still doesn't have a complete CMYK workflow. It can't compete with Photoshop, and its authors have stated they don't want to compete. And until something comes along that can, people will continue to learn and use the proprietary option first and foremost.