You want your browser to be fast. Browsers are often the only thing running. You have a bunch of ram. Unused ram is pointless. Disks are large and fast and

You want your browser to be fast. Browsers are often the only thing running. You have a bunch of ram. Unused ram is pointless. Disks are large and fast and are used for swap. It's not clear why you're suprised, what the problem is, or why anyone would put any effort into optimising a browser for memory usage. The number of people who have tens or hundreds of tabs open in the real world isn't as large as it is on HN and other tech sites, and for many people who do just keeping the url around so the site can be reloaded is probably good enough.