It's not just graphics, it's everything. Look at Frostbite for example from EA. While a good shooter engine, they had to make significant changes over the years to adapt the engine to other genres of games, and that significantly bloated timelines. Jason Schreier has reporting on this if you're interested.

We kind of forget that a lot of custom engines aren't universal ones like Unity or Unreal that can adapt to anything you throw at it. Custom engines are largely hyper specific to the type of game they were making with it, everything else is experimental, untested, undocumented, or non-existent!