It's a tragedy of the commons. You try to implement that in your browser and launch it, and many websites will break (banks, in particular). Users will downlo

It's a tragedy of the commons. You try to implement that in your browser and launch it, and many websites will break (banks, in particular). Users will download your browser and find that it doesn't work on [FAVORITE SITE X], and they won't blame the site, they'll blame your browser. After all, [FAVORITE SITE X] worked with the browser they used previously to yours; clearly, it's your fault that the site doesn't work now. So we continue to make incremental changes and work around issues with popular sites needing to sniff user agents, while authors of new sites (that someday become popular) continue to work around issues with popular browsers by (ideally) checking the DOM in JavaScript, and (sometimes of necessity because the data of interest isn't available through the DOM for security or design-hole reasons) sniffing the user agent.