Apart from the job security and steady income, if you have been a high profile character in a soap for many years you effectively become typecast and have little chance of decent TV work if you leave. Most who do leave soaps after a long time seem to end up in regional theatre.
Agree, a lot of younger Soap stars really start to believe they are something special and leave and disappear! Quite honestly in today's Job market, I can understand why an actor would hang on to a part. Reality TV and cuts in production of TV programmes etc makes the situation worse. I believe one of the actors from Corrie who was quite a big star at one time is now working in a Pub!