Having worked for McKinsey for six years, including five years interviewing for multiple practices, I can assure you there is no way this occurs intentionally.
Recruiters work hard to shortlist candidates they are excited about. They invest time and effort into getting those candidates ready. I invest my time into interviewing. Do you think any of us want to turn you down? And furthermore, leave you with a bad feeling about McKinsey people? On the contrary, we want you to be successful, so we can hire the best people.
There is no reason for us to pressure you into making mistakes or not performing your best. We treat you on the interview like we treat you once you have the job. With support and encouragement so that you'll do your best.
Any potential employer that mistreats a candidate should be publicized. In fact, as an interviewer, I could expect an extremely severe reprimand if I would mistreat a candidate like that.
Hope that reassures you. Happy to answer more.