[lucb1e]

> why can’t we just run it on their machine and propagate any game state changes (if the script adds an inserter, for example,)

Because that's an unbounded amount of traffic. You can reliably write data into RAM at many gigabits per second, whereas network connections are variable and many of them won't carry more than a few kilobits at the 99th percentile (note that you roll that 100-sided die like 30 times per second, so "1% situation" lag spikes are something you'd run into constantly)

I sometimes use Lua commands in single player to clear biters from a certain region for example, which removes many entities. Propagating those sorts of changes on multiplayer (or take a more plausible example: wave defense that eventually spawns loads of entities at once) would cause a big lag spike if you have a few players that all need to receive this data, whereas simulating it locally on each machine is no problem

Factorio multiplayer bandwidth is like a dozen kilobytes per second from what I remember, and this post agrees https://forums.factorio.com/viewtopic.php?p=125328#p125328 (couldn't quickly find an exact number though it must surely be out there). If you make it O(n) for every lua-touched entity in the game, it would quickly balloon into the megabits constantly and many mods would just not be viable for multiplayer for most people