People that question the existence of Fuchsia need only remember why Chrome was created. A lot of people thought Google was wasting their time by building a browser, including Eric Schmidt, and look how that turned out. Now, I'm not saying that Fuchsia will have the same success as Chrome, but it's clear that they think that having an OS that they can control the direction of is important to them.
Then what about ChromeOS? Do you think they're trying to build a "full-fledged" OS for the modern era, with safety and security in mind, but not as lightweight as ChromeOS?
ChromeOS was a locked down userspace on top of Linux. Fuchsia gets rid of the Linux.