The current Amiga copyright owners are remarkably litigious. WinWorldPC tries not to step on any toes (this is why they refuse to post XP - see the big red notice here [1] - even though it's beloved, relatively useful, and long out-of-print and out-of-support). AmigaOS wouldn't stand a chance.

[1] https://winworldpc.com/product/windows-xp/final

I was also surprised by the omission, but then realised they're hosting archives of the installers.

Much of classic AmigaDOS is in the Kickstart ROM (with the workbench environment loaded from disk). I'm guessing that as an OS it's too tightly integrated with the hardware to make sense in this context?

Well, the legality of making copyrighted software available for free and even providing serials really makes it a warez site. They see to be trying to skirt the boundaries of the law by mostly providing software images from disks of software from defunct companies pre2000.