Shared code repositories for OSS are much older than npm. See at least maven central for Java.
What makes npm a more relevant target is a much smaller standard library and thinner dependencies, leading to more chances that a random project will have code of suspicious provenance. The vast majority of what a backend Java project depends on was probably written either by the same 3 companies, or large, established OSS repos. So this is relatively solvable if people in steering committees decided they cared. It's improving, but I'd argue not fast enough.
What makes npm a more relevant target is a much smaller standard library and thinner dependencies, leading to more chances that a random project will have code of suspicious provenance. The vast majority of what a backend Java project depends on was probably written either by the same 3 companies, or large, established OSS repos. So this is relatively solvable if people in steering committees decided they cared. It's improving, but I'd argue not fast enough.