Making a new engine isn't something that is inexpensive. In fact it's very expensive.
I'd much rather them use a modified Unity and focus our money on the more important details of the game.
Also Unity seems incredibly versatile and have seen some really good looking games using that engine.
And it'd be even cheaper if they used a modified version of their own internal engine (no royalties/licenses to pay and the software engineers who made the engine to begin with are on site to provide technical support).
But that is only an option if it's easy to get it running on OSX and Linux. That is not an issue with Unity.