Why restrict that reasoning to browsers on iOS? You could apply the same reasoning to restrict OS X to only run Safari. Or only allow iTunes for playing music,

Why restrict that reasoning to browsers on iOS? You could apply the same reasoning to restrict OS X to only run Safari. Or only allow iTunes for playing music, since there are standards for encoding audio and organizing playlists. The fact there are browser standards is not a reason to only have one. Competition among browsers leads to improvement in standardized areas like scripting and rendering, and browsers can offer a lot of other functionality that benefits users.