Here's why:

* Carbenes are highly unstable species with a divalent carbon atom. This means the carbon atom has only two bonds, leaving two unpaired electrons. This makes carbenes incredibly reactive, seeking to form additional bonds to achieve stability.

* Free radicals like the methyl radical (CH3•) are also very reactive, but carbenes are even more so due to their divalent nature.

* Other forms of carbon, such as graphite, diamond, and fullerene, have stable structures and are far less reactive than carbenes.

While carbenes are highly reactive, they are also very short-lived. They typically exist only for a fraction of a second before reacting with other molecules.