There’s a lot of redundancy between class files in Java and zlib only has one feature for that and nobody uses it. It would require coordination that doesn’t really exist.

For transport, Sun built a dense archive format that can compress a whole tree of files at once. It normalizes the constant pool (a class file is nearly 50% constants).

Many Java applications run from the Jar file directly. You never decompress them. But you also only see something like 5:1 compression ratios.