Because it is all "vetted" by a registered accountant which is taking liability if the reporting is not don the right way. You can cut on the monthly fee for VAT/salary etc. by doing it yourself but if you consider that it costs you 80€ per month and that you invoice 100€ per hour of your work, it is not worth your time to have worry with this stuff.

The end of the year book stuff is a fee based on your turnover, it is regulated by the german accountant association.