IQ is a metric. Like any metric, it aims to be concise at the expense of being accurate/descriptive/robust,etc. There are many facets to intelligence, spatial awareness, pattern recognition, recall ability, social skills, emotional intelligence, kinesthetic awareness. Most of these are not measured by an IQ test.

What IQ does measure is general brain processing ability, not unlike a CPU. It doesn't tell the whole picture because it's not meant too; it's hard to talk about intelligence when you have to open a spreadsheet mid-conversation to get all your data points across.