Why is the aircraft boneyard in Tucson, Arizona still active?
If the number of planes waiting on getting gutted isn't decreasing even though scrapping activities are happening that means there is a steady influx of end-of-life planes arriving to get scrapped. 

As long as there are planes and new planes being built (most have a lifespan of a few decades) there will be planes that need to get scrapped.

You can't just dump the plane on a landfill or you will get nature activists on your case about the hazardous materials (hydraulic fluid, remnant fuel, ...) and recycle potential. Plus most components (instruments, engines, the espresso machine in the galley, etc) will still be usable and airliners will want to buy them as spares.