I think it might be because of the tech level thing you mentioned. Marines are basic units available right off the bat, while Banshees require a bit more work to produce (I don’t recall their tech level though). So, it makes sense that they’d have that advantage over basic units like Marines without upgrades. This is what I think though, not necessarily the thought process or reasoning of the developers.
Finally a response. Here’s the thing though, to make the starting banshee, you need a temple. You also need a summit, which actually takes longer to build with 45 seconds rather than the usual 30, this also applies to the unsc air building. Now when banshees first start out, they start at the same level as the marines when they first come out. However, the moment the banshee becomes available, players are given immediate access to the boost upgrade for a cost of 20 seconds and 200 resources. While it’s true marines can be made without any tech, even without kiting the starting marines will have a difficult time with boost banshees because they are a higher tech. Boost banshees are stronger than regular banshees.
In order to get new blood, you need to get a reactor and pay 200 resources and wait 40 seconds, not 20.
I figured with such a large amount of time needed to get them upgraded and with how slow they are, I figured they would at least be able to fight off air better. They also don’t have the advantage of instant transport like the covenant do, instead having to rely on pelicans to accomplish the same thing, which means having to spend more resources. Normally this is compensated by the eco bonus and needing to have only 1 reactor to make the unit as opposed to 2, leaving room for an extra supply pad.
So to get the marines prepared still takes a lot of time if you want the upgrades. I’m just wondering why they gave marines such a small range. They can actually throw grenades farther than they can shoot.