In a way, making it cost alloys would make armies start to taking resources that mostly goes to fleets. It would made spam armies a little harder, because even at 2:1 alloys require a additional layer of jobs to be made.
The problem is more of common sense than gameplay reasons, but i understand your question. For me armies are to easy to spam, this would make a little harder.
Right but is spamming armies actually a problem? Would having "taking planets" draw from the same resource pool as "taking systems" cause more problems?
Ground combat in Stellaris is always going to be bad, because in order for the big space-game to function it has to be abstracted down a *lot.* And that's fine, ultimately, because Stellaris is primarily about, you know, space ships, and diplomacy, and grand-scale galaxy-spanning conflicts. Ground Combat's basically just a necessary but vestigial feature meant to make planets harder to take than empty space. Having it be as simple as possible, and not interfere with the actually vaguely in-depth space combat too much, seems to me to be the right call.
As far as actual in-universe reasons, terrestrial construction requires less refined materials, and that includes arming dudes and putting them in a trash can with an engine on it. Ergo, armies just require the abstract "Minerals" resource instead of "Alloys."
