Why are long interview processes so common?

In a lot of firms, it is due to disfunction.

People are so scared to make a decision they punt and get another set of interviews.

And if the person is good, will either be turned off or have found another job in the interim.  

Because the damage of a bad hire can be so extreme that up-front investment in reducing that risk is entirely justifiable. A bad hire is almost invariably worse than no hire at all.

The increase in the number of the hiring companies internal stakeholders, who participate in the recruitment approval process, is the primary reason for the lengthy interview processes. This gets further compounded when the initial reach out to the candidate, is by a third party company which is the outsourcing partner for the hiring company.

In general hiring, the wrong candidate is more costly than taking extra time and resources i.e multiple interviews to get the right person. Sometimes this can backfire and the candidate can accept another offer so it's really important to decide as soon as you can if you'd like the candidate and make a move when you discover a pearl. 

It is also important to think about it not just from the candidate point of view, but also from the hiring organization point of view.  You are not the only person they are interviewing, and scheduling can be difficult.  I agree 3-5 interviews should be the maximum - but they may be considering 3-5 people for the position.  One individual they want to talk to isn't available for an interview for 2 or 3 weeks out - that will be a "delay" from your perspective in them making a decision for 2-3 weeks.

I think it also depends on the level being hired as to the ability for the right people to have enough free time to interview.  I find the new world of virtual interviews really helps speed things along.