Why does theme’s reset CSS have higher precedence of plugin’s CSS?

In the style.css of the theme Twenty Twenty-One, there is a piece of reset CSS:

/**  * Reset specific elements to make them easier to style in other contexts.  */ html, body, p, ol, ul, li, dl, dt, dd, blockquote, figure, fieldset, form, legend, textarea, pre, iframe, hr, h1, h2, h3, h4, h5, h6 {     padding: 0;     margin: 0;     -moz-osx-font-smoothing: grayscale;     -webkit-font-smoothing: antialiased; } 

Why does it have higher precedence than the CSS in my custom plugin? The result is that I have to add !important to override this.