Resources most likely. Training data, training a proxy that trains the real model, hardware, time, money. Managing such an open source project by itself would be terribly hard, considering the nature of model training, training data collection etc.
If lots of companies would contribute money, it would be cheaper for them to use an open model than being milked by some vendor. And what's even more important, they would be able to customise it to fit their business needs and use cases much better.