The main reason why employees leave companies can be on lack of incentives or rather lack of growth. Some of these companies are too rigid to the extent that its workers have less time to spend with family or even time for themselves. Some work six days in a week at at least 10hours daily. Another thing that prompts some workers to seek greener pasture else might be because they are not comfortable with shifts allocated to them, lack of growth within the company like raising in ranks easily like government owned work places. It could also be due to fear, some companies scare there workers with sack and they feel threatened so when better jobs opportunities come they turn to jump on it. Some of this companies also don't have retirement plans like pensions and workers tend to look for pensionable jobs since its hard to save for some during their active working days. Irregular transfers of employees by employers can effect the switching of jobs or people seeking to be independent.
Every salary earner looks and hope towards a wonderful pensionable life after years in service and this hopes can hardly be fulfilled with private establishments, for a very few of them tend to meet up to this expectations from employees and that's why every employee with a good retirement plans in mind will definitely switch from one establishments to another in his/her best interest as these private establishments are there for their own best interest too.
In third world countries especially, the only establishments that obviously renders gratuity and pension to employees after their years of service is done with are the government owned establishments and that's why many people scout for employment is parastatals and establishments own by government as the feel secure about their pension and gratuity benefits in their old age under government establishments as against the private ones. Cause a private establishments is profit oriented based their capitalist drive which is not solely of a focus from government that seek to just making sure her citizens are gainfully employed, perhaps the reasons for job switch by many.