What is the reason that most second interviews end with the hiring manager interview, considering that the interview saga normally starts with them and they are in touch with the candidate all along with way?
The other interviewers may have uncovered something that the hiring manager wants more details on. The interviewers may have also received potentially contradictory information, and the hiring manager is looking to reconcile that. If the reviews were relatively positive, the hiring manager will want to do a bit of a sales job to reinforce how the company matches your goals. If the other reviews went poorly, well, sometimes you won't even see the hiring manager, who suddenly becomes "busy".
Because they have all the information. They're privy to the reviews that everyone else gives on a candidate, so they can put everything together and make a better decision. They're fully aware of other candidates reviews and discussions as well so they have the big picture. It's their role.
You're not usually the only candidate.