Also it is not just making a game engine but you also have to build some kind of a editor, script/animation tooling, plug-in support, etc.
Basically if you are big enough that you can make your own engine the engine also has to be usable by non programmers making the actual content for your game.
Tools can be nearly as important as the engine. I've read several postmortems where the lack of good tools made the asset creation and testing pipeline take way longer than necessary, leading to delays and crunch to compensate.
Tiger Engine that Bungie uses in Destiny supposedly took hours to rebuild a level and didn’t do incremental builds or cache so fixing that one bad light on the map was potentially days of wait.
Still better than earlier versions of Frostbite Engine from EA Dice that took week if no cache was present and ran on server farm that took raw assets together with metadata and spit out final game assets.
Basically if you are big enough that you can make your own engine the engine also has to be usable by non programmers making the actual content for your game.