Why not do that? Coming from the other side of table, an immense amount of engineer-hours are devoted to the interviewing process, including code review on take

Why not do that? Coming from the other side of table, an immense amount of engineer-hours are devoted to the interviewing process, including code review on take-home assignments. Thus we try to make the process efficient. Typically there's a set of unit tests provided in the assignment to help the interviewee, and a hold-out set of unit tests that gives us a score for basic correctness. The score, albeit a low bar, is a useful threshold for eliminating candidates unlikely to succeed in the process. Those above the threshold have their code passed on to an engineer to review beyond basic correctness, and move on in the interview process.