I'll tell you why ...
They want to get everything built-in because they're trying to turn
the browser into the OS. So on their browser-OS (which even has
separate processes now) they can now run a PDF reader and a Flash
reader. They need these things running *in* their browser-OS, not
outside of it. They want everything layered on top of their browser
and completely under its control. Then they have the reliability they
need to make a browser that will never need to be killed from the host
OS, i.e. you no longer need host OS access.
Makes sense?