true revshare program will set cookies to unlimited time.. so even if your surfer visit site directly after a year you will get credit..
most of pps sponsers dont sent cookies at all.. or set it maybe for real short period of time..
this means you will never get credit for sales which happen next day after 1st click etc etc.. so i guess they get lots of "direct" sales.
and dont forget about upsells etc
there's a lot of ways to make money on your surfers.
sponsor will never pay more then what he earns himself