It isn't like the org makes a decision to give them severance after they announce their resignation. It gets written into their contract from the beginning. The reason why organizations offer large severance packages is to attract good candidates. At good companies, even junior devs get severance. In good times, it's a good thing to do. Wouldn't you be more likely to take a job if you knew that no matter what happened, you would have a nice severance package to help you find your next position?

Also, there is bloat in non-profits, but it isn't like everything is done by volunteers. Volunteers don't run the servers, they don't handle donation processing, they don't respond to lawsuits from people upset about how they are portrayed on Wikipedia. Then all those employees need HR, IT, janitors, etc., and then there are executives who run the organization.