Well that's disappointing :(, I would have expected the full Photoshop experience to be ported to the web instead, not in Apple's locked garden. Sigh.
At some point you really need to manipulate image data (from/to disk) in weird ways that's just not possible in a browser
Try opening a .jpg file that's bigger than available memory in PS and see how that goes (harder now as memory is not so constrained) as opposed to other softwares
Given we have WebAssembly and friends nowadays, you can think of the browser as a X86 VM host if you whish, with similar limitations (= no relevant limitations for running PS).
If what you said was true we would already have late Xbox 360 and PS3 like games running in the browser...with acceptable performances on high end hardware, which is clearly not the case.
I was working in the acrobat team till last month, and we were working on porting the Acrobat Reader to the web. When I left it was in the internal beta stage with minor UX tweaking work left. It is expected to be release very soon.
I think similar efforts may be (or will soon be) ongoing in other teams as well.