So that all the employees and major stakeholders don't dump stock on day 1 and tank the price, often permanently affecting the company's perception in the eyes of investors
I don't think it's solely the "price-tanking", it ties them to the company for a while.
i.e. If I was an investor looking to buy, I'd be a bit worried if the dev team would all be made instant millionaires on the day I bought in, with no incentive aside from salary to carry on working. You might find you'd just bought shares in an empty office.