Why do people think that I am so nice, and that I won't do anything bad? Like if someone hits me, they think I won't do anything. Why is that?

There could be a number of things causing this perception... Precedent (how you've previously acted) and your demeanour (the vibe they're getting from you) are two possible reasons.

If you're concerned that people think you're meek or vulnerable, and are taking advantage of you, then I would recommend worrying less about what they think, and focusing on building your own self-confidence.

It then becomes less about being someone you think they'll respect, and more about learning to be proud and comfortable with who you are.