Because to go through plane like that they need to match our solar system speed relative to galaxy.
Why should I believe some object was _intentionally_ thrown here? Maybe it is just one of those random rocks.
Sorry to be pedantic. But space is really, really, really... empty. That's why the best name for it is, space.
We live in a patch of space that's not that empty. Maybe that interstellar rock floated from other patch of space that's not that empty all the way over here, all on its own.
Most rocks we see in our patch of space, as far as we can possibly know, were not intentionally launched.