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
Back in "the day" you could have a toilet that lazily filled up and you only really needed a mild flow rate to send everything for a ride because once that 5gal of water starts spinning it's taking everything with it no questions asked.
You need a big hole to dump the tank fast to get the maximum kick out of that federally regulated amount of water you're allowed to dump per flush.
Plumbing fixture and residential water consumption regulation in the US is a textbook example of "should be a states issue". The feds basically let the desert states dictate everything and then everywhere east of approx the Missouri river has to suffer through washers that don't wash clothes the first time, constantly clogging drain traps, sewer lines, municipal mains, reduced septic performance, etc, etc. None of this is necessary in the nearly universally surface water consuming east. They'd all be better off using more water and having to size their water treatment plants up a notch to handle it.
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
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