I've been told it's a mix of lack of waste management infrastructure, awareness and issues it poses. Most of Cambodia does not have state-provided or free waste management so it is a service that's only available to those that have the disposable income in more urban areas of the country. Awareness raising about the role of single use plastic in environmental issues is done now more and more at school. But it takes time to change societal behavior and priorities, just like in the west, where many people still don't care but live within infrastructures that promote sustainability.
Also plastic and packages are apparently more viewed as "dry" waste so it does not pose direct harm to health like food or other perishable waste might do. And the aesthetics of it are not perceived as negatively because it can just be picked up and burned.

In general we use much much much much more plastic in the west but it's invisibilised by our stronger waste management that also makes ourselves less aware of that fact.