By sweeping each wallet to a different address at different times and sending them into mixers separately there is some plausible deniability that there were multiple hackers.
Sending them in blocks to the same addresses ties them to the same person or group.
It doesn't matter if they don't get caught. It does matter if they do. Legally it improves their chances of shorter sentencing with stolen coins that weren't tied to them waiting when they get out.
