Why do they want to do surgery?

I bent my thumb on my right hand backwards on 8/27 at work. The boss waited a week to fill out an accident report, so on 9/2 I saw a doctor who put it in a cast like thing and then on 9/8 they took it off and told me don’tstrain but my hand was better. I went to work the next day and my hand was aching and making me cry worse than when I first hurt it. The cold makes it hurt so bad that I want to scream.