It is becasue the available SO2 dissapates at a rate like
(arbitrarily) 20% a day. It does not dissipate at a rate of say
(again arbitrarily) 2 grams a day.

So if you put twice as much in you will just lose twice as much as
day....so over the course of fermentation you will still lose it all.

This is complicated by acidity levels and other stuff, and I did not
feel like looking up the dissipation rates...but you can get the idea.