Screen used collecting is a small niche market for a few reasons. A lot of money is spent on high end pieces but still I doubt more than a 1000 or 2000 people spend more than $1,000 to $2,000 a year on screen used props year in year out. Probably far more people spend 1000 a year on collecting toys, comic books or even posters. Also some people just can't get used to buying items that look like crap just to have something from their favorite movie if its just a stunt piece.
I imagine some people get some items from their favorite production and just get no more. Others get items selectively and display them.
Its not like comic books where some try to amass complete runs or as many as they can get. Also you have no idea what is out there. Screen used items do have a lot of crossover collecting opportunities too. From screen used comic books, to props from video game movies, screen used prop poosters, money, even baseball cards. Yet traditional collectors often pay screen used items no mind. In video games for example a nintendo world championship contest cart can sell for $5,000 and over 100 were made. yet the same people that woudl buy that would not think at all about buying a prop suit used in super mario brothers the movie or something even though its far rarer.
Except for original art and those types of collectibles, every other collecting fields items are quite common. So common people demand the best condition at the higher level items. Even the rare honus wagner baseball card that sells for hundreds of thousands or millions depending on condition here are 20 to 50 out there. Lots of props only less than 10 were made. So its a different mindset in screen used collecting.
Also you can't just go in a store or buy whatever you want at any time. In other fields except paintings and original art you can almost do that.
If you don't care about the condition you can ask around and find almost any comic book for sale to buy right now. can't do that with props. You just have to wait and see what comes for sale.
Also with screen used items its more complicated you have to deal with backup items, prototypes, items from cut scenes. No one in baseball card or comic book collecting is worried about comic books from the original printer plates being printed on the side for valuable comic books or baseball cards. Perhaps they should be but they aren't.
With props it is an issue.