The aftermarket supply line comes from 2 sources
Companies who supply the OEM part often also supply excess into the aftermarket stream and the same for companies that used to suply OEM
Others who make the same type of product for different assemblers and see an oppertunity in an undersupplied market ..
In both cases the part manufacturer will only market the product if there is sufficent demand for it .
The price of new engines is so low that most are considered throw away items in prosperous Western countries .
As such there is next to no market for original pistons let alone after market pistons .
A sad state of affairs and a prime example why the planet is rapidly heading towards the extinction of humans .
Up until B & S going bust they were making enough money from new engine sales to be able to retail replacements parts at cost .
This made them popular for the end users so the demand was high and the slim profit margins were fine .
Now that their new engin sales have plumeted to less than 1/2 of what they were in 2000 the new engine profit is not enough to subsidise the parts side of the business.
Thus for the past 5 years B & S parts have started to reflect the true price of a mssive parts inventory .
On top of that 2 bankruptcies hane cost their suppliers a lot of money so many of them no longer supply B & S or if they do it in on COD basis at a higher price in order to recoup the money they never got paid .
Thus the prices are soaring.
Since WWII we ( and I include Australians in this we ) have been brainwashed on a daily basis that the superior capatilist demand economy will produce every thing we want , when we want it at a price we can afford , and to a large extent we took the bait , hook line & sinker
So a long answer for your question but in short no demand = no supply
Short supply = high prices
And B&S is in a desperate fight for it's survival which it is loosing which is why they are expanding the logistics side and distributing more of other peoples products rather than making their own
You might find some OS pistons retailed by the companies who specialize in copetition engines but they are likely to be machined from forged blanks which are even more expensive