Much of it is also an amalgamation of licensed software. They can't release what they don't own. And remember, we're not talking $15/year startup prices here, we're talking $0.50/unit or $2 million/year licenses -- you want to bring open source to hardware, you need to deal with changing that industry. And that industry charges so much because they get "certifications". And certification costs.
And for the big guys, a few hundred thousand phones is a good customer, but nothing worth changing your business over. Not when the DoD just ordered a batch of 15 million.
The FAEs I've worked with from most hardware manufacturers have been pro open-source.... but their software departments always seem powerless and anemic, despite being the front-line for making sales, and unable to really push for it.