If I keep a local encrypted password file and copy it around by hand, I may have some vulnerabilities, but it's not worth a hacker's time to steal only my accounts, and I can probably protect my credentials from casual malice. On the other hand, if I put my passwords on the same service as hundreds of thousands of other people, that's a huge jackpot that attracts significant hacking interest, and the service only has to screw up once. The risk doesn't seem worth the convenience.