Doesn’t want to wear underwear

My daughter is 6 and refuses to wear underwear and go on the toilet. The few times she did wear underwear, she just went in them. I am embarrassed to say I kept buying pull ups because I am tired of fighting with her. I keep asking why she doesn’t to, but she just says I don’t know. I am getting to my breaking point. I am not buying her anymore. I let her pick out underwear, but even that doesn’t work. If anyone has any suggestions/advice, I’d appreciate it:)