Once you get past a certain size, streamlining matters very little. Furthermore, energy fields provide streamlining on all the smaller vehicles. The smooth, soft and almost organic curves of the vehicles, space craft and technology are purely aesthetic. They are used and found everywhere. The Blackwater is an example of a cheap, simple craft that is the very bare minimum of what is needed for space flight.
Vehicles are mass-produced to fit as many colonies as possible. There are some colonies where a Zipper is inappropriate but all the vehicles work equally well everywhere. As for similarity to modern vehicles, the way I design these is to sketch out the core components (generators, grav engine, seats, drinks machine, turbines, intakes, compressor, grav plates) and then wrap them in a shell. I don't mind that they look similar to today's cars - there is enough about Icar that is jarring for the cars to be familiar.
Thanks for the links, I follow both those and concept robots too. My aim is to make Icar individual with a basis in its own quasi-science and I find that if I start copying other people I'll end up with glowing engines on the backs of my space craft!