the monero market is not big enough to accommodate such a large amount of $
2.8 billion market cap. So a quarter the size of bitcoin. Not sure why you think that wouldnt work.
For the sane reason thieves don't just hand everything over to a fence? They know better than to stick their stolen earnings on a place where other thieves can steal it.
There are p2p exchanges that are legitimate and actually decentralized. They can easily convert their traceable bitcoin to untraceable Monero over a few years and they’ll have enough to live off of for years.