The root of it probably has to do with their sign-on options and how much it would cost to exercise those options. If you get 30–60 days to convert the options after leaving, you get a nice shiny Golden Handcuff to stay with the job until you can vest all the options and then actually exercise them. If you can do it all tomorrow? Maybe that’s all you need to allow yourself to get recruited by another company.
These are people making 300k+/year in total comp, lets not act like the same rules of living apply here equally.