That's because differentiating illicit from legitimate is exceptionally difficult on e its in the fina coal system. Most illicit money becomes legitimised in as little as 1-2 "hops". Its not like finding money stashed in a wall which has no business being there.
Source of funds controls at almost all institutions are weak by design, however making them stronger is typically impossible due to not having the ability to scrutinise other institutions controls on the matter. Ie funds go to a crypto exchange, get traded, then deposited into a transactional acc. Th3 bank that owns the transactional account can't scrutinise the exchange. They have to trust that the exchange did their due diligence on the source and are limited to asking their client what the source is, to which the answer is "trading crypto", which basically checks out.
Rich people only really need to 3xplaintheir wealth if it is sudden. And even then, only maybe once or twice. After that, it's smooth sailing.