The interview is used to weed out "red flaggers" and identify high-level strengths and weaknesses. It is completely non-technical. We assume that the person applying for the job has the technical requirements. (We also make judicious use of references, something people seem to have forgotten about. Quick phone call to previous employers can save you tons of money!)
The 3 month probation period is the real "test" and the applicant will be put on a small project with another employee mentoring. If someone lacks technical ability, it will be evident in the first few days and we can move on to the next applicant. If the person doesn't like the work environment, they can walk away no questions asked (maybe the commute is too long or they don't like the office space, etc. etc.).
This is cheaper than having directors and executives try to plan around all sorts of interview stages. It is much less stressful, as the applicant, once into the probation period, is treated like a regular employee and has much more opportunity to "prove their worth". Stop adding more shit to the interview! Don't make it longer, do the opposite!