The official tie-in toys for TMNT, Transformers, Star Wars, and innumerable other cartoons, movies, and other properties tended to be cheap plastic garbage that you'd strap on with elastics or baseball-cap snaps before running out to play with your friends. And for that purpose, it was fine; it did the job, albeit with a modicum of unsatisfaction.
That unsatisfaction was the gap between what was in your imagination, planted there by the cartoon or the movie or (in the case of the above Power Glove advert) the marketing, and what was available to you on the toy store shelf. You knew this could exist—you'd seen it! And here it was, but it wasn't that, not really. But it was what you could get, and it functioned well enough for playing pretend.
Some folks never really left that unsatisfaction behind, and became a generation or two of cosplayers who took up sewing, woodworking, plastic crafting, electronics, and more, and in some cases really pushed boundaries of materials and techniques to make what they'd seen on the screen real.
In some cases there was never a real counterpart to that. Animation was the obvious example (an artist can draw anything, whether or not it can physically exist in 3D space or with extant materials). In live action, the real production might have used half-costumes where only the part that the camera could see was crafted (for actor comfort, movement, or budget reasons) and the other part might never existed at all. If you thought you saw it, it was a rougher costume used for wide shots.
(This also comes up a lot with props. Han Solo's blaster was a famous example—there was no singular prop for that, so fan recreations were syntheses of a lot of different camera angles, working drafts, etc. Indiana Jones's fedora was, IIRC, just an off-the-shelf hat; when they started work on Crystal Skull, they actually bought fan recreations of it because where else were you going to get Indiana Jones's hat in the 21st century.)
So in a lot of the more fantastic works, a complete real costume or real prop never existed. Live-action had partials for detail shots, roughs for wide shots, and never any singular object that matched what the viewer thought they saw. Animation rarely had real objects at all, by definition; artists might make models to use as reference, but those may lack the ornamentation of what gets shown on screen, like the familiar artist's mannequin that can model poses but not clothing or makeup.
There were also issues of functionality. A detail costume might only be used for a static shot, and so not need to accommodate movement. If the actor is sitting down, the costume might not even need to be able to support its own weight. Different detail costumes might be used for different shots, and so have elements in costume A that aren't in costume B and vice versa. Props are often non-functional—at least one of the Han Solo blaster props was literally just painted carved wood.
What the cosplayer achieves, in making a complete costume or prop that (as closely as is truly possible) matches what's on screen from all angles and at all levels of detail, and makes things exist in real space that might have never before existed outside of imaginations, often goes beyond what's produced for use on set, or makes things real that you might have assumed impossible.
Small wonder some cosplayers end up going into costuming or prop crafting. A skilled cosplayer has built things that were previously literally only imagined. I have to imagine some of those job interviews contain an unusually high proportion of “you built what?”.
