Why do my friends call me ugly?

This could answered on several levels.

Firstly it could be argued that you have answered your own question. People are bullied because they are ugly.

Secondly bullies attack people who are different from themselves and who they see as vulnerable.

Thirdly the question itself could be considered bullying because you are assigning the label, "ugly", to some people. Describing someone or something as ugly is subjective not objective.