Well, some specs have been around for a while, but nobody implemented em. For example, I think the first version of flexbox was being proposed at least 10 years

Well, some specs have been around for a while, but nobody implemented em. For example, I think the first version of flexbox was being proposed at least 10 years ago, but it took a few years for it to get picked up. If I recall correctly, the initial grid spec was supported by IE10 and IE11, and those were released quite a while ago. Making big changes on the web is difficult because it requires multiple independent parties to agree and it cannot break backwards compatibility. So you need to define how it all interacts with all the legacy stuff and additionally consider how things will evolve. Another issue is that CSS has many poor defaults for applications, so developers end up fighting it often. There's ongoing work to make things better in the future by allowing you polyfill things. Check out the Houdini spec stuff, which lets you define custom styles, layout, etc.