25+ year old engine that is held together by tape and everyone who wrote it is probably long gone. Zenimax needs to either buy a new engine and hire developers who know it or clean house completely and invest in people who can build a new engine.Some of their recent published games are actually ok, such as Prey. But their own games...how the hell do their elder scrolls games have so much bugs even after so much unofficial patches?
1 They have decades worth of proprietary tools to work with gamebryo/creation.
2 They have a team with decades worth of experience using gamebryo/creation.
3 They've been hiring engineers to work exclusively on the engine for years.
4 Only a minority really get upset with gamebryo/creation. Most people are fine with it, even like it.
It'd be insane to change engines.