[gh02t]

For one thing RCT1 was famously written in hand tuned assembly language, but the main reason AFAIU is coupling of components. Early Factorio isn't that bad, but later factories it has to constantly simulate every machine on the map all of the time because everything is running regardless of your presence and everything is interconnected. That complexity scales up rapidly as your factory expands. Games like RCT aren't as dynamic and tightly coupled, so the simulation can use a lot of simplifications to cut down on on CPU use.