Well, think about this. Suppose you're running RAID-1 with two drives, and you've got some filesystem (maybe ext4, but that doesn't matter) running on top of that. You create one huge file, and then a little while later you delete it. And right after that, one of the disks dies, and you replace it.
In this case, your RAID layer doesn't know that most of the data written to the original drive is junk, and that the only really important bits are some inodes and directory entries consuming a few MB near the end of the disk. It has to re-mirror the entire drive from the original to the replacement before they are in-sync again and you are fully protected. Even with modern drives, that leaves a large window of time that you're not protected.
If, on the other hand, your RAID layer has a thicker interface to the filesystem than just a dumb block store, it can just mirror the little bit of metadata, and within seconds you're in sync again and fully protected.
That's just one example. There are many more. Go read the stories about people complaining about RAID-5 and RAID-6 performance.
Yes in theory you could probably come up with a weird pathological scenario where a monolithic design is slower and a modular design is faster. But that usually doesn't happen.
Usually, turning a modular design into a monolithic design for a tiny performance gain turns into an epic disaster/mistake. Maybe the whole ZFS thing will be an exception. Probably not.