Even if you design only one project. You still have to evaluate them, have a discussion, etc. That takes time. It doesn't scale to hundreds of interviews per week.

It's absolutely doable to evaluate one hundred take home tests in a week. It is a full time job though.

That will only work briefly because of glassdoor interview posts. However that might only mean tweaking it slightly every week or two.

Glassdoor wouldn't really help someone cheat in this situation. It's not a quick answer, it's a code discussed with the interviewer after finishing it. It would be fairly easy to tell whether someone had copied their code.