Why do alkenes undergo addition reactions but alkanes do substitution?

The difference comes from the bonds in each. Alkanes have only single C-C and C-H bonds, which are strong sigma bonds and are saturated, meaning every carbon already has its full set of four bonds. With no spare bonding capacity, the only way to bring in a new atom is to swap out an existing one, which is substitution (for example, methane plus chlorine in sunlight). Alkenes have a carbon-carbon double bond made of one sigma and one pi bond. The pi bond is weaker, exposed, and electron-rich, so it readily breaks open to let new atoms attach to both carbons without removing anything. That is an addition reaction (for example, ethene plus bromine). In short, unsaturation and the reactive pi bond drive addition, while saturated alkanes can only substitute.