Even with a problem that's unique to my organization, I don't know how I could trust that the actual candidate themselves was the one who completed it, and that they completed it without unfair assistance (e.g. from other people). Similarly, I have a habit of looking over someone's resume and picking a few random technologies they mentioned to discuss. I'm surprised how often candidates claim experience with a technology and can't really describe what they've done with it or discuss it intelligently. The level of dishonesty is high.
For all of the downsides of interview-based hiring, at least I know what I'm getting (modulo error bars on assessment efficacy).
> I couldn't remember/figure-out-on-the-spot the iteration condition for estimating square roots by the Newton-Raphson method, and I was not willing to cheat by looking it up on Wikipedia while I was on the phone
I don't ask the kind of questions that require obscure knowledge. I just ask questions that you can problem-solve with regular rational thinking, with questions that usually admit a "naive" brute-force style solution that can be improved upon. If you did want to look up some algorithm that would be fine with me, but I am more impressed by a quick and confident reasoning through the problem followed by a fluid implementation of the naive solution, than I am of sophisticated solutions with better algorithmic performance or accuracy.