They started out with the Cryengine and pretty much made it their own in all but name. By now it's able to do stuff that none of the other big engines is capable of and when they finished their meshing system, they have basically build an incredibly solid and specific base to build their game further on.
Taking another engine and just using it as is wouldn't have been a possibility with what they are doing with this game.
There are many tech video floating around that show pretty mindblowing stuff - even if not all of that transitions to a "wow" on screen.
And the systems are build in a way that they can use it pretty much wherever and they interlock with each other in a specific way.
Having a planet that has actual clouds that provide a weather system and real time shadows that are visible from the planet and from space is mindblowing to me.
Add to that biomes and gravity and the new flight model and pressure etc. etc. It's insane compared to most other games that don't try to simulate all of this but fake around it in an (quite often convincing) way.