#H_2O# forms a tetrahedral shape due to the two lone pairs. Why can't this rule be applied to all molecules with lone pairs?
Because we do not describe molecular geometry on the basis of the orientation of the lone pairs.
You are absolutely right that the lone pairs are stereochemically active, and influence geometry, however, the lone pairs are simply along for the ride.
If you consider the chloride ion,
Sequentially substitute the lone pairs with oxygen: