They’re not supposed to be able to be fired for no reason. But now with trump all rules have been discarded since the constitution’s “checks and balances” mechanism has collapsed.

I didn’t go into all the details, but besides the civil service protections, many of these jobs require extensive training to learn the job (some are quite specialized) which is a significant investment. With some entry level positions, it’s not uncommon for people to take a government job, get training and experience and then quit to go to the private sector for more money. This way the govt at least gets a certain return on their investment in training the employee. The incentive to stay for less pay after that is that after a certain number of years, you’re vested meaning you get a pension, not to mention that theoretically you’re doing something worthwhile serving your country instead of a for-profit corporation.