Because it's designed in a way that doesn't always allow these bells and whistles.
If I were to tell you that we should have kept using horse-drawn carriages, and that any of the fancy bells and whistles that cars have could be added to horse-drawn carriages, you would probably look at me with confusion.
> Improving a dominant (but broken) language is called "innovation" and "improvement"
Sure? But modifying an existing (and for most of its history proprietary) language can be a difficult, potentially-political move. If I have an idea on how to do, I don't know, linear-types checked at compile-time, I wouldn't have any idea where to begin adding that to the javac compiler. It might be easier to build my own little language to build it, and then hope that maybe the Oracle devs decide to pick it up.
There are some improvements that can be made however (any language can be improved on), that would require minimal breaking changes (like fixing Type-Erasure), and I'd even be fine with minimal breaking changes that break backwards compatibility every 5 years or so.
The problem is when people say "Let's burn it all down and start over, so zero existing code is salvageable."
I mean, let’s go with your example with type erasure; people have been complaining about it for more than a decade, and Oracle still hasn’t fixed it. The Java language designers aren’t stupid, and I’m sure they’ve read the complaints, but it still is almost universally agreed upon to be a broken feature of the language.
It’s almost never clear if “language feature X” is a good idea until it’s been implemented and battle-tested. If I have a new idea on how to build something in a compiler and I’m not sure if it’s a good idea, are you saying that the best path forward would be for me to make a PR to javac instead of building a proof of concept language?
I actually do agree that as software engineers, we're often a bit too eager to reinvent wheels. While I am not a huge fan of Java, if I owned a company, I would probably be more likely to use a JVM language (probably Clojure) than I would to use something like Haskell or Idris, precisely for the reasons you've discussed.
My overall point, though, is that often times languages themselves lag behind the state-of-the-art; there's been a lot of progress made in language design, and Java (and a lot of other languages) can feel crufty in the process. Sometimes the wheel really does need to be reinvented...if we could somehow convince the entire industry to use Lisp, this would (arguably) be a non-issue, since they allow you to abuse macros relatively easily and add language features (see CLOS or core.async for examples), but Lisp hasn't really taken over the world like I wish it would.
When you look at the millions of man-hours that have gone into the "burn it all down and start from scratch" languages (Go, Rust, etc) and you consider what if those guys had written new compilers for JAVA syntax? How great that would be. They could still accomplish many Go and Rust objectives without having to completely invent their own non-Java-like syntax. Even something that was "Java-like" is better than burning it all down every time something new is needed.
However, in a sister thread I think you mentioned that you are basically alright with languages targeting the same VM, which I think is probably a better path forward for a majority of use-cases. Clojure and Kotlin and Scala all benefit from being more-or-less fully interoperable with each other; as a result, one can feel free to experiment with language design to their heart's content without too much fragmentation.
That said, I don't know that it's entirely fair to completely criticize Rust on this; Rust exists specifically to address issues with C and C++, languages without garbage collection, and whose design doesn't quite allow the same level of compiler safety and goodness that Rust does, though to be fair Rust does have C FFI so it's not necessarily always reinventing the wheel either. I mean, I agree with the blog post we're chatting on top of; Rust might be super awesome for systems-ey stuff, but for anything TCP-or-higher, I think a managed language is kind of better.