Hiring is a two-way hard problem. On the company's side, they have conflicting interests where they want to hire someone as quickly and cheaply (cheap as in not spending hundreds of man hours interviewing just to fill a role), but also want to do due diligence so that they hire the right person. On the candidate's side, they also have conflicting interests where they just want a job and don't want to spend multiple entire days doing interviews, but they also need to do their due diligence to make sure the job is actually something they want.
This almost necessitates spending a decent chunk of time with each other, but not too much. The balance that most big tech company's seem to go with is 6-7 hours total in interviews for each candidate (and then ~10+ additional hours for both the candidate and the company doing preparation/debriefs). I really don't know why or how this was the number arrived at, though. From my perspective as a candidate, even after 6-7 hours of interviews I often come away still knowing very little about what the job actually is. And from my perspective as an interviewer, I know that requiring so much time from internal employees serving as interviewers is draining and stressful. It seems like it ultimately comes out to a lose-lose, but for some reason it's still what big tech sticks with.