The "thank you" emails seems to be a tradition/expectation of the classic corporate culture where the employer has a extreme amount of candidates for each opening and each candidate must please the interviewer at all cost. (e.g. a extreme imbalance in power between employer/candidate).

Quite contrary to the "startup" culture where there is a limited amount of candidates for each opening and where it's acknowledged that candidates might get multiple offers (e.g. a more balanced employer/candidate power)