Question

I wonder why all the men in the old time paintings are wearing white wigs?

Answer 1

To fit in. To comply with the prevailing style.
In times to come will people look at old pictures and say I wonder why the youths all seem to wear their trousers on their hips an not theirs wastes. Didn't they have belts? Did they have a penchant for flagellation? In short style, fashion.
The story goes that when Thomas Jefferson visited the "court" in Paris he entered without a wig and was admired for doing so. It's a fun story and an interesting insight into the battle between compliance and trendsetting.