Turnover

I remember asking the pastor of the mega church I attended for 12 years about the reason why church turnover of employees was at such a high rate?  He explained that most youth pastors only last about 6 to 9 months and adult ministry pastors only last 3 to 5 year on average. 

My question is why?  Is it part of the church growth model?  Maybe the church employees bring in friends and family members and then when they leave a percentage of the people they brought in stay?  There must be a reason, because if it's all about numbers and getting people into seats, it must benefit the church some how.