They make the final boss complicated and lengthy for the challenge of it and because people like it. I can’t speak for everyone, but I like the complex end boss encounters for several reasons.
They help me remember the raid later on.
I remember end boss fights for a long time because of the story elements, the unique mechanics, and because they take us a long(er) time to learn/conquer than the other boss fights in any given raid.
For example, the Azshara fight at the end of the Eternal Palace raid is the only one I remember from the entire tier, but I remember it because we had so much fun as a group progressing through it. We still have a video capture of the stream our raid leader was doing on the night of our first clear. It’s like that with a lot of raids. We remember them fondly because we remember the final boss encounter.
It is rare that I will remember an entire raid / every encounter once a few tiers and years have passed, but because of the complexity of the end boss encounters and how unique they are from one another (usually), I remember those. Jaina? Denathrius? Razsegeth? I remember those fights because they were complicated, unique, and took us a long time (compared to the other fights) to learn.
End boss encounters require a particular kind of cooperation
Raiding is fun for me (and for a lot of raiders) because it is cooperative. It’s not something you do alone; it’s something you do with other people. Everyone has to learn what to do, where to stand, how to manage cooldowns, where to pop this trinket, etc. For our group, we typically have 12 - 15, and that means hours spent laughing, accommodating, and struggling with each other until everyone gets it and the boss dies.
It’s fun.
A fight that requires 10 solid minutes of coordinated effort with 12 other people who are concentrating on the same thing that we all had to learn together over several weekends feels good. It’s rewarding. Learning those encounters together is a lovely and truly fun way to spend an evening. The complexity of the fights is necessary for that to happen.
The storytelling is better with complexity
I mean, that part’s self-explanatory. The Jaina fight, for example, told a story that everyone who fought through it will remember. The Denathrius fight is a good example of that, too.
Edited to add:
We only raid heroic, and Gallywix was incredibly easy compared to previous end boss encounters. Undermine had a distinct character and look to it, so I’ll remember that raid, but other tiers with underwhelming or “easy” end boss fights quickly fade for me. I don’t remember them because we didn’t have to work very hard to learn them. They’re over for us faster, and because they weren’t hard, I don’t remember them.
To me, that’s a wasted tier. Other people value different things about raiding, so other people will have different opinions, but that’s why I lament an easy end boss. I won’t remember it later, and so it becomes a wasted raid for me.