Because it requires acknowledgement of our weaknesses and faults. We tend to think that acknowledging such things about ourselves will cause other people to think less of us and give them an opportunity to take advantage of us.
When we’re being humble, we’re laying ourselves bare and asking others to be empathetic. And we know that not everyone will be. Some will call us weak and try to take advantage us. But we sometimes forget that having an exaggerated view of ourselves leaves us vulnerable to making mistakes. We end up hurting ourselves.
And although there will be people who will take advantage of us and hurt us when we exercise humility we’ll also gain many allies.