Maybe OpenAI has the right to not reveal anything about their research and algorithms. But why don't we see similarly powerful truly open research backed by public, universities and companies? A truly open research will benefit lots of people and businesses.
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.