There are plenty of fantastic drop in fire control groups for the AR-15.
The standard AR-15 fire control group is not worth the price it takes to get a great trigger break. In order to get a great trigger break, adjustablility, and safe, out of a stock trigger is an expensive proposition. Re-cutting the hammer, sear, disconnect, and tapping the trigger for adjusting screws, and surface hardening and parkerizing and polishing run the cost up to a good drop in.
Drop in triggers already have the correctly configured angles and adjustment screws and safety adjustments. Well worth the price.
If you are wanting a great drop in at a reasonable price, you cant go wrong with the Rock River Arms two stage trigger. Works great out of the box.