Because they made the design choice to stop making large breaking changes to the language and tooling. Java 8 to 9 wasn't easier than Java 8 to 17 is, it's getting off of Java 8 that is hard because they made the choice to break so much in 9.

Node does not promise indefinite backwards compatibility, which is a design choice that they've made that allows them to shed old baggage, the same way that the Java developers chose to shed baggage in 8->9. Neither choice is inherently better, but you do have to understand which choice a language's designers were making during the time window in question when you go to run it later.