Due to different language design. At least for the present Java does not have value types. Although apparently they are planned. Nor does Java hava pointers. Whether that is planned or not I don't know.

Without these kinds of features in the language design it would be next to impossible to implement all the strategies I have remarked on here in the compilers and runtimes.

Some of this is also simply a different choice in tradeoffs. Go desginers have emphasized having very short stop times over higher throughput.

Then of course there is the concurrency model. Changing that in Java would require a language redesign or new features added to the language.

In general you cannot just tack on arbitrary features to a compiler or runtime without having a language designed for those features.