i will never understand scientific fraud

I'd say there are three pieces, each feeding into the next.

1. A Culture Favouring Novelty Over Replication - There are no Nobel prizes for replicating findings. There is no Fields medal for roundly and soundly refuting the findings of a paper. There is no reputation to be built in dedicating oneself to replication efforts. All incentives push towards novel, novel, novel.
2. Funding Follows Culture - Nobody wants to pay twice for a result (much less thrice) especially if there's a chance that you'll expose the result as Actually Wrong on the second or third go.
3. Publish or Perish - Scientists have material needs -- both personally and for their actual work -- acquired through funding. That funding demands the publishing of novelty. If your results aren't novel, then they won't get published (not anywhere that matters, anyway). And if you don't get published (where it matters), then you don't get funded. And if you don't get funded, you perish. And so the circle of scientific life is complete.

At every step, the incentives involved in the production of science are, ironically, rewarding un-scientific behaviour and ignoring -- if not outright punishing -- actual science. Until replication is seen as an equal to novelty, this regime will persist.