In February month after the launch of FH, we had 10.828 = tenthousandeighthunderedtwentyeight new registered users.
An admin or admins would waste half their lifetime just trying to go through all registered users last year alone, imagine how much resources would be needed from the server to clean all the inactive accounts out even if it was done on auto by the server. And if not that, knowing exactly where to draw the line of which accounts that will stay and which that will go.
But I do believe you guys can imagine how long it would take to process something if lets say the server would need to delete all accounts that hasn't been used in the last year.