Because there is no user-controlled local storage under ChromeOS, at least as described now. But even if there was, no matter how much local storage you have (and the assumption is that you'll have more on the cloud and your cloud dataset size will be larger than local storage) you'll still want to make explicit storage decisions based on the availability of bandwidth and connectivity.
Say today at the office I worked on a few documents, shot a gig of video at the office holiday party, bought a couple of mp3s. All were stored on the cloud. Tomorrow morning I, before I leave for a disconnected mountain cabin, I fire up my netbook and tell it to sync. I don't have time to wait around for the video but I specifically want the docs and the mp3s. Additionally, I want an explicit sync of a group of documents from two years ago. They're in the cloud but I have no idea whether they are in the netbook cache so I want an explicit check. The only way you'd stop doing this sort of thing is if you're more-or-less always connected.
Say today at the office I worked on a few documents, shot a gig of video at the office holiday party, bought a couple of mp3s. All were stored on the cloud. Tomorrow morning I, before I leave for a disconnected mountain cabin, I fire up my netbook and tell it to sync. I don't have time to wait around for the video but I specifically want the docs and the mp3s. Additionally, I want an explicit sync of a group of documents from two years ago. They're in the cloud but I have no idea whether they are in the netbook cache so I want an explicit check. The only way you'd stop doing this sort of thing is if you're more-or-less always connected.