I think you guys might be missing some pretty obvious things here...
1. every world is going to be slightly different, but a few things about habitation are that people will tend to want to build ways to move lots of goods over long distances fairly quickly. This 'lots of goods' includes everything from commercial goods to military forces.
2. if dropships are rare, or hard to get ahold of or tasked with something higher priority, most military planners will have SOME other option.
3. 'other options' can include everything from 18 wheelers to Trains, to barges, to boats, to jet aircraft, depending on where and how far you're going to need to move those things.
4. Anything worth stealing/capturing or destroying is going to have infrastructure to move troops into, onto, or next to it if they aren't already inside.
This means there are a HUGE number of options available besides 'load onto a dropship and pray'.
most of those have already been mentioned, as a matter of fact, but keep in mind what killed the GDL. Grayson loaded up the unit on a monorail and it got nuked.
this suggests that rail transport probably is a thing. (More to the point, we have rules for building some).
The important thing for your campaign players isn't necessarily how the Opfor got there, but making sure you have Opfor for them to defeat.
for story-writing purposes, or for the really cool in-depth campaign designers who want to give their players a full-on simulation experience, it's a matter of cracking open the books and designing/generating/discussing all that infrastructure (and maybe using it to set time tables or restrict how many thillion infantry can actually respond to the raiders-aka how long to mobilize a serious counter-attack/defense?)
Really, all the suggestions here are good. Seriously good, some of the questions and objections are also good. But really, past the breaking eggs and mixing stage, what you bake into your scenario, setting, plot, or storyline really just need to be internally consistent with the game you're running or the story you're telling. (and, of course, have some resemblance to the ruleset if you're including scenario work for actual gameplay).
