Those who work anti-fraud for platforms know that there is something like a market for fraud. There is a market clearing quantity of fraud that will be present on any platform, dictated by the gains from learning to execute the exploit, the effort necessary to do so, and the number of times one might be able to do it.
Small platforms have way less fraud because they are way less valuable to exploit, in general. And that's caused by _the very fact that they are small_.
The big platforms actually are radically more competent at anti-fraud / anti-spam / etc, but the scope of their operations make them such juicy targets that there is a lot of it anyways.
It the entire world of fraudsters against Amazon, or Google, or Facebook. That's a highly unequal battle and it's surprising how well the platforms fare, all things considered.