Cause people wanted to write their front-ends and back-ends in the same language, for various reasons (code sharing, economics of hiring developers, etc).

My issue with JS (particularly but not exclusively server-side) is less that it has some weird aspects and more that its built-in functionality/standard library is incredibly barebones in many respects. IMO that’s the main reason why it’s considered normal in the JS world for a project to pull in hundreds/thousands of largely un-vettable dependencies.