Because the gas ones are not tuned for a stock vehicle. That's a good thing if the car is too softly sprung like most Fords/GMs. But our Datsuns have a good spring rate already for street use.

If you want to change the design and race it or lower it, then certainly a different damper is called for. Go for it!

KYB sell the strut insert cartridges, so it will be cheaper than getting new struts.