The current job interview process has created their services, use them until there's a shift in the interview process.These services found an unexploited niche and exploited them succesfully.

As for forgetting what you learn, yeah that's expected - if you don't use a skill it rots away.most industries don't require a rigorous knowledge of EVERY data structure and algorithm.Ideally companies wouldn't use them but if they don't too many low skilled applicants slip through the net and have to be eliminated as costlier stages of the hiring process