Why we don't hire programmers based on puzzles and tricks

So the question is what is the value of a skill or knowledge that fades away after a month or two? I know people who got hired by Google or other big companies that forgot all the information they acquired before the interview. Cause most of it is useless for the actual job. Many logic questions or puzzles are well known and people tend to know the answer before the interview.

If I had to interview someone; I would assign him a project from the company that is part the of the job he is going to do, leave him alone in a room with a computer for a time period I expect someone to finish that specific project. Finally, I come back to see the out come.

It is good for the applicant, as there is no pressure on him (except some time constraints maybe which is not a big deal), I do not put him on the spot and he does not have to deal with his boss while answering to a question. He does not have to dig unrelated information before the interview, and he can show off his programming skills if he has any.

It is good for the company, as I do not have to spend a day interviewing and prepare for that, I will know right away from the code if the guy is a good fit or not, I can see his programming skills, I know ahead of time how long in average it takes for an internal person to do the job, so I can measure his performance relatively. I do not even have to evaluate the code in front of him, I can invite multiple candidates at the same time, put them in separate rooms. Get the results, evaluate them and invite them for the second round if I liked their code.

This depends entirely on the quality if the test. I suspect for many of these ad-hoc tests thrown together for an interview, the quality will be low and the overlap with other tests high, meaning they can easily be passed by someone who googled 'top 100 programming interview questions' and read through them.

Comparing strings or reversing linked lists are not actually things that most programmers would/should ever do themselves or even nowadays bother to learn or remember outside of a CS course, because the code has already been written in every standard library, but they are things that programmers might still be required to learn in order to pass interview tests, and you would thus be testing at many your candidates on whether they had memorised a few of the problems likely to come up, not on if they could work them out on the spot.

"Of course we do not write code on whiteboard everyday. Of course we do not write code to detect if a graph is acyclic (my interview question) on a daily basis. Of course we do not put our engineers on the spot with these types of questions. However, when the time comes, may it be only once in 10 years, we know that we have hired the right people because these questions show your thinking abilities - not if you know the answer or not."

Btw, I didn't get an offer, but I'm going to Amazon where I had very similar interview process as Microsoft.

It's entirely possible that your contract has no such clause, but they are disturbingly common.