I don't know why people keep acting like PCs are "scruffy ex-military types with assault weapons in their baggage". That's one type of group, obviously, but I don't know that they'd be looking for working passage on ships. Sounds more like they'd be looking for mercenary assignments.
The Stainless Steel Rat, Nicholas Van Rijn, John Falkayn, Pyanfar Chanur, and lots of other characters who are not scruffy soldiers are great character archetypes. Merchants, spies, con artists, fortune hunters, off all sorts make good PCs. My players very rarely are former Army or Marines, though Navy shows up moderately often. They certainly aren't spending effort trying to smuggle military weaponry around.
Anyway, you can imagine the future however you want since we have no idea what it is actually like. Traveller adventures routinely present "hired crew positions" and working passage as normal. You are under no obligation to include that in your game if it doesn't make sense to you. But it is absolutely an assumption of the setting as written.
As far as the adventures go, most of them assume the characters are more willing to do dangerous stuff (like safaris or entering interdicted regions) than a normal crewman. Others presume the PCs are able to present themselves as reliable freelance hires. Some do use PCs because they are more expendable. There's a variety.
As I said, you certainly wouldn't expect that a large group could routinely get working passage, though a particularly transient crew structure is not unheard of in sci fi if you wanted to go that way. PCs would have to have some association with whatever tramp spacer/hobo culture you had going, but that's what chargen is for.
The thing is that players aren't going to use the same strategy every time. Sometimes they are going to take an iffy job that includes travel to a destination they want to end up in, sometimes they are just gonna buy a ticket. Sometimes they'll trade in favors or hire a scruffy ship in some hive of scum and villiany. The point is that it is not difficult to travel the stars without personally owning a starship.
Edit: The other thing is that Traveller treats low berth like it is completely insane to do with like a 1 in 6 chance of your character waking up dead. Which is obviously contradicted by the fact that practically every ship has low berth and reasonably expects to fill them. If you treat lowberths as more like Aliens style hypersleep, that also becomes a more financially feasible tranport system (since it's not a cattle car system used by hobos that unscrupulous crews are grudgingly taking for extra cash like the Dumarest stories that concept comes from). And "You wake up from cold sleep into an emergency is always fun once in a while".
