If forever chemicals are forever why do I have to dump this shit on my lawn every year?

I mean, are there strategic places to buy property within a development that the runoff from the surrounding properties will naturally compensate? Similar to that silly argument about a vaccination buffer zone or whatever they said.

Or, is there a saturation threshold? I don’t get how they sold me on the idea that the chemical is forever, but I have to buy some every year cause it’s gone.