I can sum that all up in these words: it's all about the money.
Duh, yeah.
But I hate the 'It's all about the money' statement because it fails to consider that there are very varied forces at work when it comes to how money works in the movie business. It basically oversimplifies.
In this case, I'm not just talking about 'money' in general, but what I was trying to imply was that merchandisers LIKE the promise that what they're merchandising will spawn sequels because the assets they sink in the first batch of merchandise can be recycled into the development of future products.
For example, development of Spider-Man movie games was significantly assisted by the fact that the later two games were merely building on existing engines that had been given graphical face lifts for later consoles and better PCs.
Of course, the those games were built on existing technology from the PS1 era of Spider-Man games, but let's pretend the first game is where they sink in the assets of developing an engine.
In any case, merchandising requires development and development costs money. If you have a franchise, you spend less money on subsequent designs because many of the initial hurdles had been overcome.
Even toy designers will tell you that, as they no longer have to figure out how to make the toy 'work' or how the figure is designed to reflect the unique properties of the character in question when they design for succeeding installments.
To reduce it all to just 'money' as if that somehow explains all the unique cases to which money applies, is a generalization that disregards that the particulars.
It's masquerading the lack of an explanation underneath a broadly generalized one that does no explanation at all.