Generation Voltage for Wind Turbine

Choice of voltage in a system is always a trade off between reducing resistive losses, difficulty of insulation and power semiconductors. I don't know the parameters for wind generators, but 600V to 800V systems are widely used in rail traction motors and generators, track-side supplies, marine electric transmissions to name just a few applications. It must be a good compromise voltage for it to be used so widely.

Yes, they uses a voltage as high as possible but are limited by the semiconductor power converters and switches "widely" available. 660V-700V is about as high as you can go with 1700V-2200V semiconductors.

The higher voltage semiconductors and their associated components are much more specialized and expensive.

It is also more dangerous to work on a windmill generating 4KV than 690V. 4KV is a different ball game with special procedures and tools. The saving of a few KG of copper between the windmill and it transformer is not worth it when you eventually need to crank the voltage to transport levels of 25KV-500KV.