Tech companies struggle to retain employees because of overwork and poor incentives (financial and otherwise) for staying with the company.

There's a reason why Steve Jobs wanted an anti-poaching agreement with Google; when people find out they could make more money working at another company, they're likely to consider leaving or trying to negotiate a pay raise.

Another problem is that a portion of Apple's compensation package (like those of some research labs, game companies, etc.) is paid in intangibles (prestige, coolness, blessings of St. Jobs, etc.) rather than dollars.