Many soap actors are far from being the greatest actors in the world. They know their profession is a minefield and by staying with their roles, the longer term cast members pretty much have job security for life and in the current financial climate, they're wisest to stay put.
People like Anne Kirkbride, Helen Worth, Sally Dynevor, etc. are from a different era to the likes of the Hollyoaks cast, who see it as a stepping stone to bigger fame, reality TV and being a WAG, probably. Anne Kirkbride apparently does the dusting in between filming scenes!