Why does a cell spend 90% of its life in interphase

Cell division is to multiply, but when you're going to perform a function--like contracting for cells, or destroying foreign agents for macrophages--then you're not dividing. Interphase describe all the time spent not actively dividing.

Interphase is when the cell grows and carries on its normal functions. Many, many cells stay in interphase all the time and never divide.

Also, the phases of mitosis don't take very long at all, so they won't make up much of the cell cycle even in cells of a root tip.