It's because of the time constraints required in an interview. At my job, the assumption is that you won't have enough context and experience to be effective in the job until you've been there for 6 months. You can't fit 6 months of prep into an interview.

Good interview questions are ones that boil that 6mo+ of experience down to something that can be explained in 5 minutes and solved in 45. Ideally you'd just work for the company for a few months and then they have their signal, and that's how internships and contractor conversions work. Most people with experience are unwilling to quit a job for a 3-month trial period though.