Yeah; the largest ones are not single vendors but whole operations with strong opsec: use of separate fully encrypted laptops dedicated to the activity, networks of "senders" that'll roam the country sending the packages from different and random mailboxes, pyramidal networks where a main vendor will sell wholesale to re-sellers (those networks also tend to appear naturally - some DNM vendors are simply selling what they bought wholesale from vendors they never even talked to), preparing the packages themselves in isolated dedicated room and with protection to make sure no DNA is present in the packages, etc. in general I imagine it probably can be a pretty low-risk (but probably not low effort) method of making money illegally, compared to traditional drug dealing. And some vendors must have seen their fortunes explode if they hanged on to at least some of their past gained BTC given the recent price hike.

But then again you also have the (from the point of view of law enforcement) low hanging fruit: vendors that encrypt nothing, keep a full record of all their customers, tend to ship with very bad stealth, etc. those are the ones that tend to get arrested. Fortunately if you're a customer it tends to be easy to pinpoint those vendors - the lack of care and professionalism tends to display itself also in their profile or during simple communications.