You need trust, competence and support.
They need to be good at everything they do. Usually you won't get that from clones.
Don't get me wrong, there are usually where a clone could be good, but the Samwer Brothers don't do it to be the best.
When shit hits the fan, you want to be sure that Stripe will step up - don't expect that from cheap clones.
Put simply, there are times when "the best" isn't necessary, for financial reasons. Particularly when we're talking about payment processors that are literally skimming a percentage of your profit.
One other good example to show how good rocket internet is: Here is a french website: http://www.adopteunmec.com/ (you can translate it by "adoptaguy.com") And here come RI in Germany: http://www.shopaman.de/, looks familiar?