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.
I love the productivity Unity brings, but sometimes I feel like making a game in the engine requires extra polish as I need to make sure the game has character and not “a Unity game”.
Is it a cliche if it's true? The difference between some slapped together product and one that feels decent is layer after layer of detail and polish. Long after you feel that you've put forth a reasonable effort, you still have half of the development left to go.
This has not been my experience.
Game developers I've worked with have generally been pragmatic and willing to use the best tool for the job.
Also triple AAA isn't that old! There's not a generation retiring quite yet :) People who want to still be in the industry are. I've seen the industry get older on average as I've worked in it.
Forms of middleware have been there since the early days of AAA style games - sound, video encoding etc. such as RAD Game Tools
Can you give some examples? I had worked on many games and a licensed engine had been always evaluated. Some went with one, some did not for technical and economical reasons. Never seen people just outright declining it because it's "heresy".
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.