^ That hardly completes the project as a durable, mobile power solution.
Yes you can just throw heatshrink tubing on something if you're going to hide it under the dash and put a panel mount socket on the vehicle, since nobody wants a cord permanently attached coming out from under the dash unless they're just trying to do it cheap and dirty and yet, it's not any cheaper in this case.
You still have to buy wire if you don't have anything suitable (esthetics can come into play here, for example if I were desperate I could take a piece of CAT5 lan cable and use it, but that pales in comparison to the best wire for the job), then there is buying the lighter plug if it's not hardwired, or the fuse & holoder if it is (or if the lighter plug isn't fused) and the plug for the other end.
It's easy to just say buy some $2 module xyz, and it's easy to do too. The harder part is the finer details of implementation in a mechanical and electrically durable manner. The same is true for most widgets one can buy on ebay/etc for under $10.
You don't really want something fragile in a passenger vehicle subject to other people being in it that don't appreciate how fragile a heatshrinked blob on a wire is. Life happens, you can't control all the variables of what someone else can do in your vehicle.
If it were the only way to get the job done, then the course is straight and the choices more narrow. When there are ready made solutions that cost less than the wire and connectors bought separately would cost (delivered), not so much.
The electronics behind things like this are much simpler, at least to those of us used to doing that, than the entire finished assembly, and thanks to ebay chinese suppliers, cheaper too.