Same reason two button mice are standard, it's the balance point between simplicity and capability. An argument could be made for there being a lack of a standard but then I'd expect there to be more keyboards trying to satisfy a market need with extra nonstandard labeling. So the bottom line is a lack of demand.
Historical reasons. Keyboards were invented long before computers, for teletypes, with 5-bit codes, or Morse codes.
That said, some national variants have more special characters; for example, French AZERTY gives easy access to § ° € £ €, etc
There are keyboards designed for foreign languages including Russian, Arabic, Japanese, Hebrew, etc. that have keys for the required characters. There are also custom key tops you can buy, stickers for the top and front of keys, flexible plastic overlays, and most OSs have key remapping software.
Ages ago, I worked at a very large PC company, and we had a shelf of more than twenty different keyboards we tested with.