Here's why:

* Strip mining is used for ores close to the surface. This is the defining characteristic of strip mining. The overburden (the layer of soil and rock above the ore) is removed to access the ore. This is most efficient when the ore deposit is relatively shallow.

* However, not all surface ores are mined by strip mining. Sometimes, the ore may be too deep or the overburden too thick for strip mining to be practical. In these cases, underground mining techniques might be used instead.

So, it's more accurate to say that strip mining is a method used for ores near the surface, but not all surface ores are mined this way.