In space-trading games like Escape Velocity, Elite: Dangerous & others, cheap FTL exists, but lags in other advancements results in a playable environment that is politically fractured and not a post-scarcity utopia. Pilots can even manage a profit margin trading mundane materials (like water) just a few tons at a time to space stations in star systems with no terraformed planets.
So in a galaxy where stellar neighborhoods can be traversed in hours or minutes, and the demand for moving mundane materials is high, what constraints probably exist that result in tiny, sometimes chintzy & fragile, spaceships being a favored form of transportation for an average licensed pilot? Why are large freighters not seen as generally superior? Or, alternatively, why is the volume of space traffic shown in this type of fiction lower than expected? After all, we just mined x-million tons of asteroidal iron and our water usage is through the roof, as we have many thirsty miners in ships not terrifically larger than an 18-wheeler.
Some common constraints tend to be that A.I. is not a pervasive technology, so robotic swarms are not usually part of these interpretations, and that atmospheric flight is either impossible or prohibited for most interstellar ships. This means ships can usually only dock on space stations or outposts on airless worlds, which are expensive & time consuming to manufacture & operate, and therefore a trade bottleneck. (these could be perfectly acceptable answers to the question, if they are the only limiting factors)
Also worth noting: the FTL drive of Elite: Dangerous' effective range for a single "jump" is inversely proportional to the amount of mass being moved, which presents a hard limit on cargo & theoretical ship size depending on the quality of the drive. Assume this is arbitrary, or that creating increasingly stronger drives isn't insurmountable from a scientific or engineering standpoint.
EDIT: I'm sorry I could only choose one answer, many of the below responses offer a lot in unpacking this common feature in near-future FTL universes. Thanks all!