The problem is, while possible that "white-board style questions eliminate a lot of bad candidates", it's a terrible and empirically unsubstantiated methodology of assessment given the purported goals most companies claim to have in hiring people.
In the event that it works at all, it does so by accident, not by design. Which is often evident in how hit-or-miss hiring someone is in most companies even after they've managed to make it through all these silly contrived crucibles.
To a certain extent it's a cargo culting problem. Insofar as companies like Google do it, and Google appears successful, and so other companies (failing to do any meaningful analysis or meta-analysis of their own) decide to model Google on the basis that surely their company will be successful just like Google. The problem, taking Google as the example, is that Google has AdWords. Which is for all intents and purposes a money printing machine. Google can be basically terrible at almost everything they do and still appear wildly successful so long as they don't disturb their money printer. As a result it's very difficult to just pattern-match to what Google does and evaluate either its benefit or its appropriateness in any context.
The other part of it is inertial. Often the wheel of hiring in a company started this way for lack of any better ideas, and then as the company grew it became proportionally more biased toward people who made it through a process like that, and then it becomes generationally perpetuated. The unfortunate side-effect of this is that biasing the employee population this way makes the job, for a new executive or manager, of changing these ineffective (and sometimes toxic) strategies extra difficult while trying to maintain team cohesion. Because it has a lasting cultural fingerprint.
Honestly, it's really a shame. I'm sorry you're having to go through it. My capacity to try to interact and change this weird marketplace phenomenon extends only about as far as the company for which I'm the Engineering hiring manager. My hope for you, and for others, is that you either find a place to work which doesn't do things this way, and/or you otherwise advance far enough in your career that nobody bothers to foist these silly ceremonies on you. Because your involvement in their pursuits is seen as a priori beneficial or existential.