I have a different take on it. As to why they don't develop software for the market along with movies. Most of the tools created in house at Weta stay there. The really cool stuff they keep there to maintain there completive advantage. For some years. Eventually it trickles out like:

Mastive, used on LOTR, for crowed AI. I suspect they have had this for about 6 or so years. They are thinking about selling seats for it for 40,000$. I really don't think that is going to be a part of Maya. For a while yet.

A good number of features in the pro software was developed on movies true. but it takes a good while for it to trickle out in to the open market. Once a studio see's that the other studios have this capability and the software companies haven't already put it in. That’s when they start talking to the software companies about selling there IP.