That's simply not true. The best language for a front end is the language that's preferred by the platform. Firefox famously rewrote their UI with Java on Firefox because the XUL version was miserable. You don't build iOS apps with Java. You don't build Android apps with Go.
You can't actually build a mobile app entirely in JavaScript, but if your used React Native the parts that actually deal with materially interfacing between the client and the server aren't in JavaScript. At best you can share some typescript definitions.
Java on the server is extremely different from Java on client, and the same can be said for Swift. What do you get to share? Maybe the code that serializes and deserializes messages? But even then that's thanks to a library, not the language (and if you're using a feature of the stdlib, it's almost certainly just as easy to use in any other reasonable language).
Pyodide and PyScrpt exist and you could certainly build a full stack web app with them. Why doesn't anyone, then? It's because doing so doesn't solve any serious problems or make anything much better.
You can't actually build a mobile app entirely in JavaScript, but if your used React Native the parts that actually deal with materially interfacing between the client and the server aren't in JavaScript. At best you can share some typescript definitions.
Java on the server is extremely different from Java on client, and the same can be said for Swift. What do you get to share? Maybe the code that serializes and deserializes messages? But even then that's thanks to a library, not the language (and if you're using a feature of the stdlib, it's almost certainly just as easy to use in any other reasonable language).
Pyodide and PyScrpt exist and you could certainly build a full stack web app with them. Why doesn't anyone, then? It's because doing so doesn't solve any serious problems or make anything much better.