In a perfect world, things would be like you describe. Unfortunately we live in a diverse landscape of browsers and devices, many of which have not been update

In a perfect world, things would be like you describe. Unfortunately we live in a diverse landscape of browsers and devices, many of which have not been updated in quite some time. Factors ranging from IT policy at older companies, to geographical limitations, may prevent many users from upgrading to the latest and greatest. Some companies choose to cut their losses, and support only certain browsers - these can get away with less compilation and polyfills. Other companies may be targeting these specific segments (such as B2B), and can't get away with such a luxury.