There's also the fact that factories are one of the buildings you can amplify; since factories and fusion reactors are manned by engineers, you'd have to make sure to never add new factories to domes with crewed reactors or your power output can drop by 20-33% (in other words, 40-60-ish power loss) per person that leaves the reactor, which means if you don't have a new dome available you need to build one, which means moving people in, which means adding more houses and servicies, as well as running cables and pipes to it, which means increasing power, water, and oxygen consumption, which can mean having to add more of each, which increases maintenance... You get the idea. Unless you have an empty dome or one not connected to manned fusion reactors, it's actually much cheaper to apply the amplify upgrade to a factory than it is to build a new one outright.