A company may value its workforce and want to keep them, in which case it makes it attractive to stay employed with them by providing good pay and benefits.
Companies such as these can be selective about who they employ and also enjoy the benefits of a stable motivated workforce.
Other companies simply pay the minimum they can get away with and accept a high staff turnover, reasoning that it costs less to keep training new people rather than pay enough to make them want to stay around.
A certain amount of turnover may be desirable because if no one ever left then pay and conditions are likely to be excessively generous, and people leaving allows for advancement for those staying.
Unions may also get a strangle hold on a company enabling them to demand pay and conditions far in excess of the industry standard. Survivable when all your competitiors are in the same boat however it comes unstuck when you have non union competition....