The idea that diecast product cars are valuable collectors items is probably true, since they are generally classed as collectibles since 1930s. In 1934, Dinky Toys presented diecast metal toy cars, which presumably parents brought for little boys, who grew up to be men who still liked the cars, and turned play into collecting.
Anyway, contrast a lot of these collectibles with Beanie Little ones, which turned out being a fad. Nowadays you see them mainly in thrift stores, not in display cases or placed on lamp stands. You do see diecast cars within thrift shops, but they are segregated and individually billed, not thrown in with other small toys for the reason that cute little baby wildlife are.
Why collect car models? Well, why collect anything? You either get it or you don’t. But collecting these little or low number of little cars does make sense to numerous people. To begin with, they may have started out as toys, but they soon developed into scale models, or miniaturized representations of actual vehicles. They are true to life in proportion and depth, thereby rising above the toy level to your distinction of true designs.
Diecast has traditionally meant being made of metal of one type of another or a mix of metals, forced into a mold while molten, with much pressure to help fill the mold and allow the metal to help harden. Today you may well have diecast plastic cars. The metal ones are definitely the collectible ones of selection, it seems, although early plastics have definitely develop into collectible, so who knows?
The models are still being enthusiastically made, and the age of the model does not discover a method to automatically raise the amount. As in the original, actual car, the appeal is the eye of the beholder. A model car may be a remembrance, of the VW bug that’s your first car, in the memories that are aroused through the sight of a 1969 Mustang convertible, or of sitting on to the floor watching the General Lee fulfill your six-year old ideas of such a car could do.
It might also be as close as you will ever get to owning a Ferrari, say, or a Lamborghini. If you really always wanted one, somebody will give you a model for Christmas definitely. If you really charge, it may be a premium model with doors that open, a steering wheel that turns not only itself but the wheels too, and a working suspension. If your friends really get carried away, they may get you one with a working internal combustion algorithm, although they are venturing near to the toy line here.
As in all collecting, it pays to focus. You may want to start with police cars, with all the versions in the Bat mobile, with cars from famous movies, or from the era of history that interests you most. Pre-owned of models in these categories and more is incredibly large, and there are many manufacturers who useful to and/or are still making these small vehicles.
It may turn into a hobby, too; you may want to put your cars into display cases and hang them to the wall, but you may want to restore them, airbrushing on bright, glossy colors, and set them in dioramas with little, diecast gangsters or carhops or service men the optimal scale to match.