For the same reason that Stuart was always involved in investigating every crime to take place in Ramsay street. It is cheaper to get a regular cast member to diversify by doing things other than their usual speciality, than it is to employ an extra to play the part of a specialist.
Also, you have to take account of the peculiar demography and geography of Erinsborough, whereby it is completely surrounded by high and impassable mountains which means that it is very difficult for anybody to pass in or out of the community. The few people in Ramsay street society are like a self-sufficient sect where they have to work for each other all within a few hundred yards, and do all the work that needs to be done, no matter how complex. Whenever anybody leaves the tightly-knit community (even to visit relatives), they are "shunned" by the rest of the tribe and they are never mentioned again.
Occasionally, outsiders stumble upon the closed society of Ramsay street by accident, and it is routine for teenagers from the outside world to be unofficially adopted by various resident households, often on very spurious or tenuous reasons to do with distant relations.