.. Because major browsers will need to integrate it and they haven't. You can build things yourself more easily than you can convince say chrome to build something (which is the whole point of extensions, really). What you seem to be asking is just metamask to be integrated directly into chrome, which is a nice ask but not done to the right people (ask google/firefox) and its lack does not turn web3 into a 'Stupid Idea'.

There's a lot of extension functionality I'd like straight in the browser itself but it's not the extension's fault that browser devs haven't included it.