The argument against eggshells is that most soils have sufficient calcium and it takes many years for the shells to break down into a usable form and this is true. There also needs to be a balance of nutrients in the soil, you don't supplement one at the expense of others. NPK (Nitrogen, Phosphorus and Potassium) are your main plant requirements and the others are just trace.
In my case, the soil acidity breaks them down quickly and the geology of the area is calcium deficient because the rocks date back to a time when there was no life on earth. Younger soils, or soils that were ancient inland seas with marine life don't lack calcium