The main issue in The Walking Dead is the fuel or gasoline (gasoil) which has a life span duration of few months.
After 3 months, the gasoline starts a degradation process (volatility, evaporation, oxydation) becoming a sort of grease and then rubber. It can't be used after 6 months. Except few exceptions (like no contact with air).
So they can't drive vehicles or get some viable gasoline 6 years after a world virus contamination.
Source : Fuel has a shelf life. The degradation of fuel has consequences that can include power loss, increased consumption, plugging, and increased maintenance costs.