[koko775]

> why not use the system WebKit framework?

Same reason people often like to statically link. The system is guaranteed to be a moving target sooner or later, so new bugs may appear or disappear depending on a user-controlled action that's tedious to track if it's possible at all.

Bundled code, on the other hand, is a fixed, known quality against which bugs can be tested, or source inspected, or otherwise debugged in a way system libraries typically cannot.