fwiw, I have a snapdragon X laptop running with windows. It basically never runs the fans (at least at audible levels), and I sometimes sit with it on a blanket in bed
what you suggest by gluing libs together is more like a framework. Engines have cohesive asset systems that tie into engine and gameplay systems, networking has to work with the gameplay layer, etc. Also, tooling is like 90% of what makes an engine worth using. You need asset importers, level editors, scripting solutions (often visual these days), data editors
I think starting in the 30s and esp after WW2/cold war era helped reset it somewhat. It started to pick back up in the 80s and into overdrive with the internet