Why should I write my own HTML and CSS over using what Bootstrap gives me? I can't think of a single reason to believe that my HTML/CSS will look or perform be

Why should I write my own HTML and CSS over using what Bootstrap gives me? I can't think of a single reason to believe that my HTML/CSS will look or perform better than Bootstrap's. Especially since you can selectively import only the components you use. You're essentially betting that you can like for like implement what Bootstrap gives you better than what they can. Which just seems like a terrible bet regardless of how good that particular developer may be.