Because we want to find the best people at solving difficult problems with limited time, and maximum ambiguity.

At a high level that reflects the work that we do. The project scale may be 20 weeks instead of 20 minutes, but the level of complexity is scaled accordingly.

Of course, we miss out on some candidates that could be great at executing on a large timeline but can't demonstrate it in a short in person interview.

But, (see my edit), this is all we can do to scale our interview process, and we don't want to take risks.