The IOC doesn't pay athletes. Their home countries and endorsing companies pay them.
Their home countries are generally the ones giving them drugs - most doped athletes are doing it under the orders of or at least with a blind eye turned by their national sporting and drugs bodies - so it would be a bit rough for them to punish them. "We gave you drugs, and you took them, you bastard!"
The endorsing companies certainly have an interest in it, but does Nike donate to WADA? Is it WADA's job to protect some sportswear company's investment? Is ASADA working for Kelloggs?
How about if your sport gets no sponsorship, is it okay to cheat then? Does anyone sponsor clay pigeon shooting or archery? If there's no money involved can they cheat? How about a 16yo running in a state championship, there's no money in that. We don't test because money is involved, we test because we want sport to be fair.
Let's just test active athletes, and test people at their events. But we don't need to retest samples from years ago. There are a limited number of tests and limited funding for it, so every test you do on someone's sample from 2008 means one less test on someone in 2016. You're catching cheats from yesterday while letting cheats from today get away with it. I don't think that's the outcome anyone is hoping for.
Again, if 2008, why not 2004? 2000? 1996? 1924? At what point do we say, "fuck it, too late now"? Again: these aren't Nazi war criminals. It's just sports.