Here's why:

* SN2 reactions require a good leaving group: Alkenes don't have a good leaving group. The double bond is relatively stable and doesn't readily leave as an anion.

* SN2 reactions require a nucleophile: While alkenes can react with electrophiles, they don't readily react with nucleophiles in an SN2 fashion.

* SN2 reactions require a backside attack: The double bond in an alkene is a planar structure, and backside attack is not possible due to steric hindrance.

Instead of SN2 reactions, alkenes are known for undergoing:

* Electrophilic addition reactions: This is the most common reaction type for alkenes, where the double bond is broken and new single bonds are formed with electrophiles.

* Oxidation reactions: Alkenes can be oxidized to form epoxides, diols, or other products.

* Polymerization reactions: Alkenes can undergo polymerization to form long chains of repeating units.