It is inert, not reactive (to the surrounding elements).
This is a very good question and the answer relies solely on reactivity.
If an element is very reactive, for example, fluorine, it will react with other elements, forming a compound.
If an element in not reactive (inert), there are some reasons why. The main reason is its electron configuration. Is it stable? How does it form bonds - polar, non-polar, etc.?
These questions should be considered why an element is found in its pure form. An example of these kinds of elements include gold - which is why it's often used in jewellery.
Keep in mind that all elements will react, but some don't react with other elements as often. This includes considering elements in the vicinity of each other and being exposed to other elements.
Hope this helps :)