Because if they don't keep records, if an entity was chasing after someone, the mixer company couldn't provide them with any info even if they wanted too
you should read terms of services and privacy policy of each mixer that use. but as far as I know they all say that they are not keeping any logs, because that would be against their purpose as a mixer!
for example here is what Bitmixer.io says in its faq section:
Absolutely no logs or personally identifying information are kept regarding your use of the Bitmixer service. All logs are permanently removed from both our website database and from our Bitcoin wallet after your coins have been transferred out of our reserve. All forwarding entries are removed after 24 hours.