There are number of browsers like IE, Safari, Firfox, Chrome, etc. And they are not relied on JavaScript ie. you are not just limited to apply the JavaScript on the browser. But they can work ( run sites/server ) with different languages like JavaScript, PHP, C#, C++, Java, HTML, CSS, etc. It doesn't mean browsers must run with JavaScript.
For example, modern Google Chrome browser is based on V8 engine which is nothing but JavaScript and WebAssembly engine written in C++.
You can now ask why only V8 engine? No. There were many browser engines:
V8 is developed with more optimal performance especially for JavaScript as you know most of the websites today run with the JavaScript. You may read and see V8's benefits here.
Hope, this makes sense now.