Actually, it's now a few but rathet a pretty big bunch of very different corporations & many of them direct competitors. And thanks to the kernel project & its license, they can still work together towards the common good.

I'm sure it's a large expense. However, they make more money from the data and market share they get from it.

Why else would they keep moving things out of AOSP and into their proprietary frameworks?

Same for Linux itself - however in that case, there isn't incentive to make it proprietary, because the companies are either making money from hardware sales or from support contracts.