Surely the UNSC would have their own energy swords by now

Yes, the Sangheli design is iconic. It’s a nightmare for human hands and probably isn’t great for the Elites, but it’s Iconic.
However, we’re decades post-war. Surely the private sector would be selling energy blades to spartans right now, just as 343 should be selling players skins for covenant weapons.

More broadly, by the time of Halo Infinite, Humans should have gotten their heads around covenant tech. Does this mean the UNSC will universally adopt plasma guns? No: A good chunk of human stuff is just more effective and practical and you’d think the Covenant dysfunctional for even fielding some of what they used. But some stuff is, canonically, really impressive: Beam rifles are lighter and more lethal than the Anti-material rifles the UNSC are using for snipers: giving them a shape ergonomically good for humans can only be a step up. Fuel rods are fun…

Also I don’t know why there isn’t a covenant carbine skin for the commando.