The most honest answer is that studios can't extend the software without violating the GPL.
Maya is outrageously extensible in a variety of ways and really set the standard for being "open" in this way. A lot of the big studios have toolsets that radically transform parts of Maya, shaping it into the tool needed for the work.
Maya is also pretty crusty, plenty of other applications do a better job at it's core competency, but they simply aren't dug into studio pipelines like a tick in the way that Maya is.
Blender has to limit itself to Python, or at least, I'm not aware of the ability to create native plugins. I'm all for Python, but a lot of interesting work is done in C++ building native operators for the application or embedding custom viewports for gamedev which I don't think is remotely possible in Blender yet.
Additionally, licensing prevents many integrations from getting off the ground.