Why don't we treat all takeoffs as short field in GA? Depending on the airframe, there are a number of techniques to reduce the Take Off Distance required. For example, deploying flaps.

What advantage is there to NOT using short field take offs, even when runway length is sufficient? Surely it's always advantageous to require the minimal amount of runway, thereby adding a larger margin for error?
Short-field take-off techniques often achieve a shorter ground run by hurting climb performance. While having more runway is great for safety, planning to use less of the runway is not a huge benefit: you always plan to have enough runway left if you need to abort during the take-off roll. It's much better to have more height quickly in the climb-out: it gives you more margin against obstacles after the runway's protected area, and it gives you more height to land in case of an engine failure after take-off (EFATO).

The numbers for a normal take-off optimise for a good rate of climb, which has the greatest benefit to safety. A short-field take-off gives you an extra option when the runway length is too short for that - at the cost of some margin if there's a problem in the climb-out.