Why do outdated billboards hang around?

This empty billboard in Jalan Damansara is unattractive and according to DBKL must be replaced with a civic message.

JALAN Syed Putra is not only renowned for its congestion, but is a highly sought-after location for billboards.

But, curiously, among the many billboards that dot this busy stretch, there are quite a few that advertising events that are long past their dates.