Back when injection molding was the only game in town, model kits were produced within lattices called sprues, in reference to the runners of an injection mold.
Producing a single mold that contained all of the parts was faster and more cost-effective than having individual molds for each part. The downside was that it wasted plastic, but we're talking about an era when adults smoked cigarettes around kids who were playing with lawn darts.
Now with 3D printing, you can just print the entire model in one go. But for those who still miss wasting plastic, there are a host of sprue card model kits you can download from Thingiverse.
I'll be impressed if they designed in the parting lines and ejector pin marks.