Some of that is due to the cost of living escalator ~ which tends to keep your federal pension increasing in numbers if not purchasing power.
Until quite recently, as pension rules go, federal employees could not also start an IRA account ~ after all, they had a pension plan so they were written out of that law.
Later with the reforms of the mid 1980s federal employees could have an IRA and also an equivalent of the 401(K) plans the private sector got earlier. Initially the savings levels were kept way down so there was little value to these plans until the late 1990s.
Federal employees were also prohibited from joining Social Security.
As I understand it, the defined benefits pension plans for federal workers ended in 1986 and after that, those who were covered in the old plan were offered something equivalent to a 401k plan, as you pointed out. They could keep what they had or go with the new plan. Those hired after a certain date in that year (I’m not sure but I think it was 1986) had no choice, they got the 401k plan and their personal savings and that was that.
But, heck, that doesn’t stop people who are ignorant of matters from fuming and fussing that government employment is nothing but one huge bed of roses.