Why do simple alkenes undergo addition reaction, instead of substitution reaction?
Because alkenes are a more unstable structure than alkanes, and it takes more energy to break off bonds that are already there than to simply form new ones.
Alkenes are unsaturated molecules, which means they do not have all the hydrogen they could have. This is because there is at least one double bond between carbons.
This is a stable structure, but not the most stable, so when certain compounds or elements are added, like fluorine, they undergo an addition reaction.
An addition reaction is when something is added on - nothing is lost or changed. For example,
You can see this looks like a very simple addition mathematically to form 1,1-difluoroethane.
When they actually react, if you picture the Lewis structures, you can imagine the double bond in the middle is opening like a gate to accept the fluorine, leaving it with an alkane-like structure of single bonds.
They do not substitute because it requires more energy to break off the bonds that are already there than to simply make new ones.