Why cant vodka be made by fermentation alone?

Vodka is a neutral spirit (i.e. without distinctive flavour) with an alcohol content of around 40%. Alcohol is created by fermentation - the anaerobic respiration sugar by yeast. Sugar and alcohol in sufficient quantities both inhibit yeast so there are practical limits on the alcohol contentration that can be produced by fermentation alone. You're doing very well if you can even get to 20% and what you produce will not be flavourless. Fermentation is a biological process and a large number of other chemicals as well as alcohol. Distillation extracts the alcohol out and leaves the other chemicals behind.