2% only applies in a large sample size such as yours.
2% chance applies to all sample sizes, it's just that the larger sample size shows a closer representation to the true chance. The chance doesn't change to 1% if you just use 100, it's still 2%.
Unfortunately, most don't roll 499 to achieve that outcome (to acquire the same amount of stones), as that would cost 12974 gold at best. So if I've only got 3k gold, I only have 115 oclays, even though probability is the same, the likelihood of me acquiring the same amount of those stones as you is unlikely.
If the chance of getting at least 1 stone in 50 clays is 63%, I wouldn't call that unlikely. Though it's true that the stones aren't always fairly distributed because the game uses a random function rather than hand out a stone every 50 uses of oclay. I did get 2 on my first 100 stack, chances are you would too.
At 2% I'd only acquire 1 stone, but at your price, with 3k I can buy 2. So why would anyone roll oclays then?
No one knows how many stones will be produced from 115 clays until they use it, you can't just assume you'll only get 1. Also, the price shouldn't be 1.3k, I simply said that it's worth 1.3k, rollers should profit a bit, but they shouldn't charge double what they spent.
Not to mention those that do acquire said stones may use them instead of selling them, making the availability in the open market even more scarce.
Likewise, them using the stones lowers the demand because they don't need it anymore
Because not everyone has 12k, nor the time, and demand is high, it's only natural that whatever is in the open market is driven up in price.
You'd only buy 13k worth of oclays if you want 10 stones, which much better than spending 26k for 10 in AH. Also, although the video is long and tedious, I only spent 15 minutes to use 100 clay. It would be faster with three sprites, so I can't imagine time is a problem.
The economy works. GF is a very simplistic model of supply and demand, the prices reflect well.
Edit: Also 2% is still a chance, not a guarantee. I'd have to roll a large sample size to get close to 2%, which would cost 12974 gold (for 499 oclays).
Whereas if I just buy with 3k, I can get two stones at your price, which is basically saying with 3k I can guarantee myself 2%...
...why should anyone roll oclays then if at less than 1/3 the gold I can guarantee even money exchanged on the 2% probability, instead of taking a risk using 12k in gold, which most people don't have nor have the time.
Don't forget that there's a chance to get 3 or even 115 stones from 115 clay, or even vit+6 stones.