Why all alkenes are hydrocarbons?

What are alkenes ?
"Alkenes" belong to a class of hydrocarbons containing C=C double bond.

Since "alkenes" belong to a class of hydrocarbons, then all "alkenes" are hydrocarbons.

Bit like asking why are all flowers plants.

By definition.

alkenes are hydrocarbons that possess a double carbon bond. it is part of the definition of the term: alkenes are hydrocarbons that...

Because they have carbon and hydrogen, by definition, along with at least one double bond.. If it has no hydrogen, like in Cl2-C=C-Cl2, it is considered a chlorocarbon.

by definition ... alkenes contain nothing but C and H .. and hydrocarbons ALSO contain nothing but C and H ...
But there is a difference also ... alkenes must have a double bond, other hydrocarbons might not. Alkenes have a general formula CnH2n