The interview requirements & assessments are driven by the hiring companies and not the headhunters.
Scaling technical interviews of this kind has created $1 billion companies recently.
Technology employers want an unbiased assessment of a candidates skills - like any other firm.
And it's exactly what drives the bureaucracy, the transfer of responsibility. All you need to do is blindly follow the request in the most defensive possible way and no one will ever be able to accuse you of screwing it up. So you have headhunters who just follow the clients' briefs, client's HR follows their playbooks to cover their asses, and as long as they all get paid in the end of the month, who cares if you miss to hire some really talented people. It's developer's mistake, they should have prepared better for the test.
And I'm not trying to accuse anyone here of being guilty in particular, just to point that the whole process of hiring is deeply flowed by the very design. On the brighter note, it motivates a lot of really smart people to stay out of big corp world and go indie, which is actually great...
Some firms provide take-home problems/projects - typically with a time limit. Their objective is to provide a more realistic scenario for writing code while evaluating the candidates technical skills.
This is so wrong. Headhunters are acting like typical bureaucrats and instead of adapting their approach to make it better at finding the actual talent (the whole point of their job), they push for developers to fit their stereotype tests better.
Just like back in the school when we had to learn what professors want to see in essays, instead of learning to actually write better...