In reality, it was not that humans could not see ghouls. However, it was not a good thing that one could see them.
Take her situation as an example. She was weak and could not get close to her family because she saw something that she should not.
Apart from that, she would be scared every now and then.
Those ghouls had no sense of direction. They could float and fly, bashing around. Now that she was getting better at talisman inscription, her health was much better than before. When she was young and could not defend the ghouls' power, she would occasionally get possessed.
Naturally, it only happened a few times.
The Wei couple looked pitiful.