Question

why a nail point tip have sharp edges and buildings have wide foundation

Answer 1

just think it, that if a building has a foundation like nail point ,than like nail building also get penetrated in the ground

Answer 2

A nail point has sharp tip thus giving it a small surface area and 'as pressure is inversely proportional to surface area thus the nail will have larger pressure on a surface thus going in a wood plank etc.

As for buildings large surface area gives means a smaller pressure on the earth surface as per the reason stated above (in inverts).