Because html+css was designed for viewing static documents, not unlike Word or PDF. Most of the efficiency problems of the web stem from that fact. The rest stem from the bloat that's required to support every browser in existence from the past 5 years. I still shudder every time I hear the words "this works in ie8, right?" It's like having 5 different Java VMs, each supporting a slightly different undocumented version of the java bytecode spec and then requiring everybody to support all 5 VMs, their quirks and make it all backwards compatible.