Another restriction in addition to other comments, a lot of companies have rules saying senior executives have to maintain a certain amount of stock to hold their position. This means they often can't sell substantial proportions of their holding or else they lose their position and have to wait until they leave the company.

Let's say they need to hold a million shares and get 100 thousand a year as part of their compensation, yes they can convert those 100k into cash, but the remainder is tied up.