It may also happen that most people are not that tech-savvy, but wise enough not to trust themselves to store their coins locally, in their own desktop wallet, where the coins will be prone to theft by viruses, trojans, etc. I mean these people are not sure that they can provide the required level of security themselves.
I agree that keeping all coins on desktop wallet especially on Windows it's risky too, maybe even more risky than using Coinbase for unlearned user, but there are a lot of solutions between Coinbase and desktop wallet which mitigate both risks. BitGo, Copay, even Blockchain.Info is better.