Because this is the status of the bug across the standard libraries of various languages, per this article and others:
Erlang (documentation update)
Go (documentation update)
Haskell (patch available)
Java (won’t fix)
Node.js (patch will be available)
PHP (patch will be available)
Python (documentation update)
Ruby (documentation update)
Notably C and C++ are missing from this list because their standard libraries don't even offer this capability. Half of these standard libraries are responding to the issue by just warning you about it in the function documentation. Rust is one of the few that actually prevents the attack from happening.
The original BatBadBut bug report used JavaScript to illustrate the vulnerability.