Coz they used to be a low volume, niche-core-tastic thing that would cost a relatively high amount per unit to produce. Imagine how many 26" inner tubes are produced worldwide compared to the number of Stan's strips…
Plus, back then they had the market virtually cornered.
Now-a-days there's a lot more options out there, but they look at how they should price, and use the existing (Stans) benchmark as a guide, and despite the fact that Stan's must be selling a gazillion more kits than they used to, they don't drop the price. So nobody else does.
And that is how the bike trade works.