They’re disempowering themselves. Describe themselves as a victim of circumstance, powerless and without responsibility. It’s a disempowering narrative. You’re shooting yourself in the foot. In this story that we live, you are casting yourself as an npc.
On the flip side, saying “I can’t” adds authority to your argument. You’re citing an unstoppable force as the root cause of your actions. So it’s a tradeoff that way.
Being able to tell people no (I won’t) is a good skill to have in life.
Some people don’t like to tell people no to avoid conflict, and they may end up in situations they didn’t want to be in.