Think about the Chrome sandbox. The website rendering process is sandboxed using seccomp, so that even if it is compromised, it can't do anything but modify the way the web page it is displaying looks. Since you could do that anyway by editing the HTML for your web site, attacks against Chrome stop being interesting. (The exception seems to be attacks against Chrome's embedded Adobe Flash, which they are still in the process of fully sandboxing).
If we did something similar for PDF readers, all a bug in the PDF rendering code could do is change the way the PDF looks-- which is not an exploit.