Many kits do not include such things as running gear and finishing products as frequently owners want to personalise the model with their own preferences. Also, of course, some may want all out performance while others might simply be happy with a more sedate speed so, in many cases the manufacturers simply do not include propellers, shafts, tubes etc..
There again I have built a couple of kits that included a plastic propeller, which was not the best performer by a long way so they were replaced with a brass one. It could then be argued that I have wasted part of the kit cost so it would have been better not to include it in the first place.
There is also the angle of course that the additional bits and pieces can be purchased over a period of time, thereby spreading the cost out, with some manufacturers, such as Mobile Marine Models, even spreading the cost of the kit itself over a number of convenient payments. Lets face it most modellers are not going to complete a kit in a short space of time so spreading the cost over the period of the build works for a lot of people.
These things should be brought to the attention of the purchaser at the point of sale but, all too frequently, it is assumed the buyer knows what the kit contains. I would always recommend that the question is asked if the information is not voluntarily provided.
Even if you scratch build from a plan you are still going to need to buy the same components and materials that the kit was short of as well as building materials, the plan, glues and paints etc. so maybe the kit wasn’t as bad a deal as it initially seemed.