Typically because i may buy 5 freighter have them for 8 months and then decided i'd rather have British Airways models. So then i just sell them, normally at a much lower rate than i bought them, and buy the BA models.
Some out of production models increase in value over the years so people buy them now and sell them 25 years later when the company is bankrupt. Such is the case with VEB Plasticart models that I collect which I have to buy from people like that that bought em 20 years ago.
Yeah only a very limited number of models actually go up in price a good amount. And those are typically samples that only a handful of ppl get there hands on or limited models to begin with that cost $50 from a retailer.