If you get an average paycheck, pay 25% of it into the pension for 40 years, you expect to get approximately 50% of the average paycheck at the time of retirement. Well.. you don't (atleast not here). This also presumes, that there are 2x as many workers that pay into the fund, as there are retirees taking out of the fund... which sadly is not the case anymore, and the situation will get worse and worse.
Also, due to many laws and changes, every few years, you get average-paid people retired in eg. 1991 that receive more than the average paycheck in retirement, and newly retired people getting 400eur of retirement (=not enough to survive).