Laziness? Plus I like the fact that it's integrated into the browser itself. That might be irrational though, not sure how well integrated plugins can be in core browser stuff.
Uh, Chromium's open source, it could ask for FTP credentials and then upload them there...? There are any number of ways this could be done without giving any information to Google and only giving them to Chromium.
How could google put your bookmarks somewhere that they don't have access to?