Yes, I wholeheartedly agree, it was a stupid decision by the Selenium devs not to make request headers etc. accessible. But why throw all the standardisation efforts overboard?
Given that the selenium leadership is apparently uninterested in improvements, and it's many limitations, trying to improve there is more effort then it's worth.
Basically, they're still stuck in this idea that they're ONLY for emulating user-available input (and the dev-tools don't exist).
In reality, there is tremendous interest in more complex programmatic interfaces, but apparently they're unwilling to see that, and are instead only interested in their implementations "user-only" ideological purity.