Plus it takes time to do the bug squashing since there's a bit of back-and-forth between testers and developers. A tester finds a bug and submits a report; someone on the dev team then has to read the report, implement a fix, and that fix has to be incorporated in the next test build. The original tester then checks to see the bug is fixed once that test build is made, etc...
It's a bit of a process. :) Even though Cobra mentioned that the code is basically complete, I bet they're still working really hard at this point. I'm sure it will be a huge relief for them when the plane releases!