You don't have to, but they reduce development cost by reducing the impact of per-device differences such as those between various versions of OpenGL ES, various versions of OpenGL, various manufacturer-specific implementation quirks of OpenGL, D3D, the existing desktop below-OpenGL APIs and now, presumably, metal.
I was being slightly sarcastic: the point is that there are people who enjoy doing both (and indie games are for the most part a work of love/enjoyment). If Mac drops OpenGL (or never adds Vulkan) there is a chance that someone simply won't bother making their engine+game work on Mac - meaning lost potential on the platform. Who's to say the next Minecraft (which is an engine+game) won't support Mac because it doesn't support Metal and instead went the Vulkan route?
Apple can port Metal, it makes sense: there are tons of mobile games that would pose immediate value for the platform; adding Vulkan into the mix (or even fixing their completely broken OpenGL implementation) would have been a much stronger message.
