There are a bunch of reasons why it's a bad thing, and most of them are compelling to me --- in principle, you worked for 3-4 years to earn equity, not the opportunity to buy equity --- but there's a big reason that I think is dispositive.

When a company sells, it negotiates with its buyer for a long time to come up with a price and terms for the deal. Former employee stockholders are iced out of that negotiation. The buyer and sellers have levers to pull that improve the outcome for themselves at the expense of former employees; for instance, the terms of the deal can include retention bonuses paid to employees, and in fact are sometimes dominated by those bonuses. Nobody needs to decide to be evil to make something like this happen! It's rational for the buyer to want to extend additional benefits to keep employees, and management is numbered among those employees.

I've had friends who paid significant amounts of money for their stock only to have it end up worth 0 in subsequent high-8-figure sales.