Flavor and logic. I used to be a proponent of the 1k ton disposable survey ships, but that was nixed by a couple things, partially a greater concern with RP, partially the drive changes and addition of crew morale making tiny little survey ships less effective, since they can't stay out for decades at a time. A larger dual-role survey vessel has more room for maintenance space and extra crew quarters, and frankly makes more sense than FAC-sized survey ships.
Because honestly, it's really, really easy to munchkin everything in Aurora. Robo-logic hyperefficient play gets boring pretty fast. It's much more interesting to create and work with a theme for a given campaign, otherwise every single one is going to proceed pretty much the exact same way.
Did I mess up some equation? Because ignoring fuels supply, this means that the more powerful engines would always be more space-efficient. That's... counterintuitive. Why the hell would anyone use the lower powers for military ships then? For anything then?
Because the fuel costs become insane for anything more than a small civilization with a relative handful of ships, and Sorium is a fundamentally limited resource (like the rest of the T/N minerals). If you run all of your ships with super-inefficient drives, you run into one or several of three problems:
1. Your shipbuilding outpaces your fuel production. You can't afford to operate all, or perhaps even most of your ships.
2. You burn through your local Sorium deposits. So you need to go farther out to establish new mining colonies. Which costs fuel, and which brings you into contact with more enemies, which cost fuel to deal with. You have to use more fuel to get the fuel or Sorium back to your main colonies, or you have to take the time and expense to move shipyards and maintenance facilities out to remote locations. As your territory expands in the search for more Sorium, you need more ships to properly protect and service it, which need more fuel.
This is a problem regardless, but with sane drive design it's a low-level one that takes a long time to manifest. If your fuel consumption is quadruple or quintuple the normal rate, it accelerates substantially.
3. You run out of fuel and can't get to any more Sorium deposits because you have no fuel. This is a risk even in normal games, but if you're jacking up your consumption there's a very real risk that you won't have enough fuel left to find, extract, and transport more Sorium once your local sources run out.
Sure, it sounds good on paper. But Aurora isn't just a game of theorycraft and paper designs, it's also a game of logistics and practice. A slight edge (and it
is slight, especially at lower TLs where drive strength is low in general) in tactical combat isn't worth completely destroying your ability to field large fleets or forcing you to push beyond your reach trying to secure more Sorium. The only way I'd ever even consider using fuel-guzzling drives as standard issue would be if I were playing a cheat game where I added something like 100b tons of Sorium to Earth.