There is a difference in a few things.
1) You are comparing OEM replacement parts to aftermarket parts.
2) These OEM parts are mass produced.

Lets not be stupid and compare aftermarket car shocks/struts to ATV struts.

The Eibach Pro Kit is over $2K and it just always height adjustments.
The Eibach Multi-Pro Kit is over $2750 and offers height adjustments and single adjustment damping.
http://performance-suspension-eibach.shptron.com/p/multi-pro-r1-coil-over-kit-single-adjustable-damping-ride-height-6


The Stage 4 Elkas offer height adjustment, high and low speed compression adjustment, and rebound adjustment.
Price is $2600 (You can get Stage 3 for $1500 right now)
http://www.elkasuspension.com/products/atv/atv-stage4/
These Stage 4 shocks are also set up for your quad, riding style and weight, not just a cookie cutter set up like the Eibach.

Now pistons

JE pistons for a SBC is $792. http://www.jepistons.com/Products/217240.aspx
Wiseco Piston for a SBC is $98.86. http://autoplicity.com/1212377-wiseco-pistons-piston-shelf-stock-chevy-sb-9cc-rdome-1060-x?utm_source=GoogleShopping&utm_medium=CSE&utm_content=GMC-G35-1212377GSMakeModelFitment&utm_campaign=GSMakeModelFitment&gclid=CjwKEAiA-5-kBRDylPG5096R8mASJABqEdm4lcUK1jPeHE5eChAPg7wnWUnjqjEsu5wCQu8GMBgJbhoCBMXw_wcB

A wiseco piston for a zilla is $100-$120ish.
http://www.ebay.com/itm/like/171140905275?lpid=82


Not seeing a difference in prices here.


So again, next time you should compare apples to apples.