It doesn't help because it gives people a feeling that they're already doing something big. That way they deprioritise the environment in their head.
Instead, just refusing a work trip or something once a year helps a lot more than using ecosia.
But it seems Ecosia does really help around the world, at least looking at their reports certainly seems so. Even if it's not a huge impact, isn't it better than nothing? How does NOT using it help anyone?