Question

Why can't we directly inject atp into our cells for energy?

Answer 1

Some issues could arise. ATP is used as a signaling molecule between cells (a recently discovered phenomena), injecting signal molecules in such high concentrations is a dangerous proposition because very small amounts are required to produce a proper response. Also, if one was to try to bypass this and somehow distribute ATP to your cells (no idea how) you would disrupt the intricate network of rections and feedback loops in the cytoplasm resulting in a momentary disruption of normal cellular function. Changes would most readily occur in the mitochondrial functioning and cell metabolism. Most likely the increase in concentration of ATP in the cytoplasm 
would temporarily decrease production of more ATP Try injecting something a little healthier.