In the case of Getty, they provide a service in many ways inferior to GIS or BIS, which kinda counts as the whole reason we have this topic in the first place - MS's cute little slideshow widget worked better than Getty, thereby completely shutting Getty out of the picture.
Anyone actually interested in paying for stock photos, OTOH, already understands the difference between freely available vs licensed content, and damned well won't risk their job "accidentally" ripping off random photographers.
Let's say I'm interested in paying for a stock photo. I go to Google, and search for my project's key terms. I get seven cats, thirteen memes, and a mugshot on the first bunch of results. I try different terms, find one I like, and... then what? Not every website includes contact information, and if they aren't outright trying to sell me pictures, I have to go hunting to even figure out where to ask.
Maybe I'm lucky, and I find a site with contact information. I call up the photographer, and he's willing to negotiate. There's a back-and-forth exchange where I offer some amount of money, and he wants a hundred times that. Forget it.
I go back to Google, and try again, luckily remembering the refined search terms I used in the last round. In amongst the blogs written by that license-lacking Grandma, there's another candidate for my project. Searching Google for that image doesn't show any other sources, and it obviously isn't Grandma's original work, so there's another wasted effort.
Finally, I hit the jackpot. I find a photographer who has posted prices, and has a decent picture that fits my needs... but he only takes PayPal payments, and says he'll email me a copy of the picture in "good resolution", whatever that means. One of his pictures looks familiar, and sure enough, a bit of investigation shows that it's a pretty common candid of an office worker, used in catalogs and on support pages across the Internet. Could it be that this guy's the silently-famous photographer, or is he just selling others' work to make a quick buck? It's a bit too risky for me, so that "jackpot" is another dead end.
I give up. I'm well on the way to spending more time on the project than it's worth. If only there were some other company to do the sourcing work for me. They could negotiate with photographers, index pictures by business-relevant keywords, and provide reputable proof that I'm actually getting a legitimate license to the material I'm paying for. All of that risk is eliminated, and the project could stay within a constant time and financial budget.
The service Getty provides is ultimately the same as any other broker: risk mitigation. They do the acquisition work, and assume the risk of high acquisition costs. They also do resale, and assume the risk of having unsold goods. Because they work on a large scale, they can specialize enough to reduce those risks to an affordable level, and I can simply pay that cost, plus a bit of profit for them, to benefit from their specialization. Getty earns that profit, and I spend less overall because I'm not wasting time on those dead ends. Everybody wins, so everybody's happy with the trade. That's how commerce is supposed to work.