halavai: when you say “all the poskim”, can you please name a few, and show where they said it? Was it in a letter? In a sefer? Please let me know.
When I was engaged, my wife took a class which covered why women cover their hair after marriage. After all, almost every rule of tznius applies before marriage as well, why is hair different? She was taught that after marriage, the hair of a woman has a certain power which it gains to attract men. It is kabbalistic in nature, and not easily understood, so I won’t try here, as I don’t fully understand it myself. However, it has nothing to do with how attractive the hair looks – otherwise it would be an issue before marriage also. Therefore, a wig which looks real but covers the hair probably accomplishes what it’s supposed to.