Here are some reasons:
AAA is by definition games that aim at the top end of what can be done in performance and graphical quality. Browsers prioritize other things. Put another way you can't be both AAA and in the browser, because if you tried, other people would come along and simply do better than you outside and you wouldn't be AAA anymore.
Specifically, browsers insist on very strong levels of sandboxing at the cost of high overhead, and they don't want you to run native code either, so you lose both performance and compatibility with most existing game libraries/engines. They also insist on everything being standardized and run through the design-by-committee meat grinder. Whilst Microsoft are polishing up the latest version of Direct3D browser makers are still trying to standardize the version from five years ago.
Browsers are optimized for lots of tiny files, but game toolchains tend to produce a small number of big files. For example browsers aren't good at resuming interrupted downloads or pinning data into the disk cache.
PC gamers have unified around Steam, which offers various advantages that raw web doesn't. Steam is intended for native apps.
Many games need to be portable to consoles because that's where the revenue is (bigger audience, less piracy). Consoles don't run web apps.
Browsers not only don't make it easy to implement anti-cheat but actively make it as difficult as possible.
Debugging tools for native code in the browser aren't as good as outside.
And so on and so on. That's not a full list, it's just off the top of my head. Other types of apps the web ignores: CLI apps, servers, anything to do with new hardware, OS extensions ... the list goes on. Really, we must ask why we'd ever think it'd make sense to ship AAA games as web pages. If you want the benefits the web brings for without the problems then you'd want a new platform that tries to learn from all of this and be a bit more generalized. I wrote up a design doc a month ago that tries to do that, see what you think:
https://docs.google.com/document/d/1oDBw4fWyRNug3_f5mXWdlgDI...