The reason they have insane stock offerings is because statistically speaking they know you won't stay around.

No, they back weight the vesting schedule because their compensation model is built around forecasting/manipulating the stock price in such a way that tenured employees will get a "raise" at no additional cost to the company (the shares are purchased and held in escrow at the time of grant--so if they appreciate by 30% over 2-3 years, the employee is "paid more" by some substantial percentage more while labor costs do not actually change). By withholding the bulk of the vest until the latter two years, the effects of the anticipated stock gain on total compensation are magnified. Large (guaranteed, not performance-based) cash bonuses are given during the first two years to compensate for the weak vest when calculating total comp.

Simple arithmetic bears this out if you look at the total compensation (as described in this thread):

103 + 18.5 + 0.05 * 65 (3.25) = 124.75 103 + 18.5 + 0.15 * 65 (9.75) = 131.25 103 + 0.4 * 65 (26) = 129 103 + 0.4 * 65 (26) = 129

It's still a cynical strategy that favors the company, but not nearly as cynical as holding out carrots in front of employees on the assumption that they won't reach tenure anyhow (that is far more common in actual startups).

Yes--it's a high risk, high reward strategy--if the stock doesn't grow as intended, and there have been numerous periods where it has, retention follows along with it. However, high turnover is also anticipated and built into the staffing model--but that's another discussion for another day.