This is at least in part because hardware has advanced to the point that for many games it makes sense to optimize for development time over runtime performance. Unity and Unreal are still not as performant as a well optimized specialized engine could be in many situations but it would probably take too long and cost too much to build that specialized engine.

There are also trade offs in flexibility that come with using Unity or Unreal. They make it easy to make games that fit within their paradigms but it's relatively harder to make a game that does something radical or experimental with rendering or physics.

I say this as someone who lives in Unity. It's absolutely the right choice in many situations but it's not without tradeoffs and compromises.