"Why do we need telephones when we already have messenger boys?" Attributed to an anonymous businessman when Bell introduced the first phones.
Well, because first you have to go to the place you're going to - say, an Engineer - then you have to order a transport, and pay for it. Then you have to wait for the thing to turn up. Then........you have to do all that again to move it again to where you want it. Really, it doesn't take a lot of imagination to see the benefit of self-transport.
Hmm, I've heard reasons, but none that I consider logical. In fact, none that I consider other than silly, tbh. This issue of volume makes no sense to me at all, even on its own terms. Apparently, an Imperial Cutter can hold an 80t Shield Generator in its cargo slot, but not a 32t Sidewinder. "Ahh, but think of the difference in VOLUME!". OK, you tell me what the volumes are of those respective items. And what about all the other various ship's components: power plants, distributors etc? No doubt you'll tell me they're also too large to fit into the cargo space of a large ship, but that makes no sense to me at all. A cargo ship should be able to carry, you know, cargo. Big and small.
I honestly don't care about volumes anyway. I genuinely believe this is just a rationale from people who are always afraid of change, no matter what it is.