Here's why:

* We live on it: We have direct access to Earth's surface, allowing us to study its geology, atmosphere, oceans, and ecosystems.

* Telescopes and satellites: While not spacecraft in the traditional sense, telescopes (both ground-based and space-based) and satellites provide a wealth of information about Earth's atmosphere, oceans, and surface.

* Remote sensing: We can use various techniques like radar, sonar, and infrared imaging to study Earth's features from afar.

While we send spacecraft to other planets to learn about them, we don't need to leave Earth to study our own!