I've worked in a regulatory agency, albeit for a brief time. I worked with analyzing data, usually in cooperation with law enforcement, as part of some case or operation.
I don't know what to tell you - but my experience was that it is the most careless and obvious criminals that get caught. Some even being very rich, rich enough to afford solid defense teams, and rich enough to stay in court for a long time. These were also the very same type of people that would cry to the media about unfair treatment, witch hunts, etc.
But, as for why these agencies go after the money: It is much easier to prove money laundering, rather than the actual crime itself. Many of cases I worked on focused on companies that illegally harvested regulated natural resources, used unreported employees, and did not report their sales / trades. Off the market, under the table.
We co-operated with other regulatory agencies around the world, and I can tell you - in the more corrupt parts of the world, the effects can be devastating on the community: No honest companies can compete against the ones that operate illegally, dire working conditions, less tax funding to the local community. What happens is that these actors eventually become local monopolies, and start operate semi-legally, but that's what the community is stuck with. If you report those companies, you end up losing your own job - should the company go down. And since these players don't care about regulations anyway, whatever natural resources they harvest, can risk being wiped out.
And to re-iterate: It is easier to follow the money. Usually in these cases, where there's smoke, there's fire.