I'll provide a different option: it takes too much time to get it right! iOS for example provides so many default widget toolkits complete with gradient, sheen, and dropshadow which make it really easy to create a polished experience quickly. The web has no such concept (well, bootstrap is sorta close but not nearly as polished and not for mobile). Why does the web not have these polished frameworks? Simple - ROI. The incentive for Apple and Google to develop said frameworks is large: a pool of developers who will take advantage of the toolkits and write much better / way more apps. What's the incentive for the web? Basically nil. Anyone can rip off what you did (one of the downsides to the web being open and standardized!), and you gain none of the benefit of enhancing your platform.