You'd be surprised.

For one thing, there are interface changes needed because lexicographical difference.

For another, they can't just copy and paste the server code because doing so would probably cause incompatible user data which means while they can start a new server with all up to date stuff, they can't do so to old server for the risk of all players losing their data.

They have to upgrade piecemeal like the Chinese server did because the step-wise upgrade process did take data preservation into account and thus would be able to preserve the data.

That said, I'm not saying that they have to upgrade as slowly as the Chinese server did nor that their current pace is their actual limit. They can certainly role out updates much faster with new features every month if not every other week. But they certainly can't just jump directly to the end.