First of all, I've always trusted Coinbase a lot more than most other Bitcoin services. They're a US company, a YC company, I've met the founders, I know some of the employees, they at least appear to be very serious about security, etc.
Second, those 80 BTC are not all of my Bitcoin. I don't completely trust myself to securely store Bitcoin, so I like having some diversification of risk.
But yes, some of it comes down to laziness. I should probably figure out a better way diversify the risk without needing to trust a 3rd party.