Obviously I won't go into any specfic things here, but as a typical example say a fully rigged up/textured car might set you say $500 to buy. Lets take the house keeping, like revisions build time as a given, as again that varies.
Now you're more than a little bit poorer, obviously you need your investment back and quickly. As my maths are cr*p :) lets say you set a selling price of $10. Thats not 50 copies you need to sell, but 100. Because you need to factor in the store cuts. 50% being the general average for most places and that covers them marketing your product, handling the customer service etc.
That can be achiveable in a month or two, however that assumes it will sell. Yes while having a good well made product and nice promos helps theres also a big element of luck involved.
Mostly you have to hope it'll be well received by your audience - I've spent 6 month plus on some stuff and (as an example here) because it's too unique it's tanked, other times I've knocked a quick freebie out in a day and been so popular the host has had to migrate my site to a faster rack.
[Note if you self sell thats also an extra cost, not just the techie costs but time to code it , promote it etc. The latter not being an easy task]
Theres also the question of will the site selling will be good with it's promotions. OK sure theres an valid argument ...more you sell, more exposure you'll get. Equally if a site releases loads of new products each day realistically how much they can promote just one person ?
So yes overall your point that ..."creators do almost all the work, assume almost all the risk, but hardly make close to almost all the money. To me that seems very unfair"... is correct a lot of the time.
Then again thats also common in the traditional art world and there you're you've got the complication of being up against those with limited talent but the cash to pay for shows, adverts etc. So why do creators do this, my feeling is it's about the love for the craft and sometimes we just need to make a few bucks to cover costs etc. It's not about making a living because thats not likely to happen for most sellers.

Pinky - you left the lens cap of your mind on again.