For the government to make the necessary modified version of iOS, they need to have or acquire a team of skilled engineers that either already have detailed knowledge of iOS and the devices it runs on, or a way for their team to learn that knowledge. This would probably be somewhat expensive in both money and time.

To acquire Apple's keys, the government only needs someone who can replace the device key and re-sign a new iOS package, and a sympathetic judge that will sign a subpoena, warrant, or national security letter. This is practically free and shouldn't take more than a day or two (warrants are sometimes granted near-realtime).

I don't understand why would think taking Apple's keys (which wouldn't require "stealing") somehow requires more resources? A warrant or nsl is much cheaper than a team of developers.