Allocation is typically really cheap, maintaining pools for objects would likely have more overhead. And while collecting garbage takes resources too, it's heavily concurrently, especially in GCs like Shenandoah and ZGC. So instead of more overhead due to pooling on the thread that uses the objects, you have more overhead on a different thread during garbage collection.

So while it makes sense to avoid unnecessary allocations by using different APIs (e.g. not creating Streams in hot paths), pooling brings far more new problems with it. It might make sense for large objects, but generally requires in-depth analysis to make sure it actually helps.

Also, when plugins come into the equation, you need to make sure that those can't modify objects they aren't meant to modify, which involves copying of objects. Additionally, some objects have different representations in the API (what's used by plugins) vs the implementation (what's used by vanilla minecraft), so converting between those representations is another source of allocations.