I think this is part of the process to ridicule candidates.

Maybe interviewers are cynical after interviewing so many really bad candidates and the only enjoyment they get from the process is the ability to look down on them.

I have done a few of these take home assignments, later to be engaged by the firm, then to discover in a firm of 100 people, I only really met one or two who could even do the assignment, let alone do it in under an hour.

I have not found this to be the case, but I think the usual idea is that if it takes longer than advertised, you should probably stay away from the job.