They do the majority of the work, so they get to decide. That's how it works everywhere. If WP Engine wants to decide they should invest the same 4,000 hours/week.
That's the problem though. It's a chicken and egg issue. Those donated hours could be directed towards things that aren't in WPEngines interest. To use an analogy: it's taxation without representation. That seems counter to the open source ethos.
If everything was handled by an independent, transparent and accountable foundation then this would be a different conversation. It isn't. It's handled by a private, for profit, vc-backed company with a leader known for personal vendettas and holding grudges.
I want to be clear: I have no love for WPE and agree they should be doing way more. I'm just pointing out that the current arrangement is not exactly conducive to facilitating that.
> Those donated hours could be directed towards things that aren't in WPEngines interest
I have a hard time seeing how that can happen. WP-Engine still decides what bits of WordPress they work on – it's just a matter of having developers who work on it.