Why are interviews harder than the job?

Companies keep ignoring the historical interview point. I have been to a few occasions when companies needed the exact thing i built in the past (e.g. migration to clickhouse), but chose to put me into a random take home related to a different technology (e.g. some bigquery assignment) and eventually reject me. Go off script and ask me details about the project which might solve your hands, why do i need to talk about something else?

quick answer: this takes time & energy; the interviewing company is willing to miss you to save this effort. Plus the interviewers are not likely to be good at these tasks, even if they're solid developers; it's a skill they have relatively little practice and training.

I understand this for a dry candidate pool. I have been there as a hiring manager, you need some signal with take home or live interviewing. But on the rare opportunity that you find someone who is willing to talk in details about a similar project you want to do, you skip the pipeline and if he indeed built it, you hire him.