I imagine such caching is slowly being phased out because it can be used to create 'super-cookies'. That is, you can fingerprint a user by detecting whether certain bits of javascript are or aren't cached. (Detection of being cached is just a matter of measuring execution time).
It _is_ cached, but the wasm binary itself as well as the optimized version to improve startup times. The cache however is per origin. So no other origin can make use of the cache which prevents the fingerprinting aspect.
