Our industry could follow after the restaurant industry and implement stages/trialing. You come in and work a full day. If you get the job that days pay shows up on your first paycheck, if not they send you out the door with a check.
I think most companies just don't want to take the time to set this kind of thing up. Easier to just pass on people that won't do the requisite song and dance.
Also I've had many dev interns at my company, it takes work on my end to enable them to be productive quickly, but it's not impossible. Maybe you need to write some skeleton code for them to fill out and unit test, maybe you need to document things well and spilt up your tasks well, but you absolutely can have some one come in and be productive day one.
In fact my interview was a college hire group interview, we were given a laptop with skeleton code in multiple languages, a problem to solve with fixed inputs. The problems were similar to what you'd likely encounter for whiteboard coding, except I got to be left alone with a computer, the internet, and my editor of choice to write actual code to solve the problem.