Here's why:

* Petroleum is a complex mixture: It's made up of hydrocarbons (molecules of hydrogen and carbon) with varying chain lengths and structures.

* Boiling point and molecular weight: The boiling point of a hydrocarbon is directly related to its molecular weight. Smaller, lighter molecules have lower boiling points.

* Natural gas: Composed primarily of methane (CH4), which is the simplest and lightest hydrocarbon. Its low molecular weight results in a very low boiling point (around -162°C or -260°F).

Let me know if you'd like to learn more about the different petroleum products and their boiling points!