Probably easier to transition to an engine members of the team have experience with as apposed to pulling people off id projects to teach them idtech. While Microsoft owns Zenimax I don't think it's as easy as just asking for whatever they want.
I figured since Blam!'s extension has been a money pit with little return on investment, why would MS continue to flirt with Unreal's royalties and licensing when they have something, basically for free, inhouse? I get Unreal's been ubiquitous. But I had a think about it. It's probably related to the contractor issue, and you're right. Anyone they pull might have some familiarity with Unreal before idTech, depsite idTech's higher permeation on PC, I think consoles are just overall less exposed to idTech or its descendants.
On that note, MS should open up idTech for licensing again.
Maybe not let that technology go to waste.