Go's stdlib is separate from the language. The language spec doesn't specify a standard library at all. It also doesn't have just one stdlib. tinygo's stdlib isn't the same as gc's, for example.
I will note that gc's standard library also isn't written in Go. It is written in a superset with a 'private' language on top that is tied to the gc compiler to support low-level functions that Go doesn't have constructs for. So separating the standard library from the compiler wouldn't really work. No other Go compiler would be able to make sense of it. go1 promise aside, the higher-level packages that are pure Go could be hoisted completely out of the stdlib, granted.
In Java, the "stdlib" that comes with the JRE, like all the java.* classes, counts 0 towards the size of your particular program but everyone has to have the whole JRE installed to run anything. Whereas if you pull in a (maven) dependency, you get the entirety of the dependency tree in your project (or "uberjar" if you package it that way).
Then we could decide on which of java.util.collections, apache commons-collections, google guava etc. become "standard" ...