Why hasn't there been a zombie apocalypse?

There are 3 possiblities:

1. It may not ever happen at all. Zombies may not exist.

2. It might not be the time. Maybe in the future, but most likely not soon.

3. It may not have happened yet because no scientist has been able to create a zombie virus.

There is a video from Vsauce about zombie bugs, which shows what could be a zombie virus.