why the unit of total surface area of cube and cuboid are in meter square although it have length breadth and height

It is not the question of number of variables in the formula.

 

Area is the measure in two dimension and Volume is the measure in three dimension.

 

Now consider the area of the side of a square.

 

Area = side x side

 

The total surface area of the cube = 6 x (side x side)

 

In this formula side is multiplied by side and there are only two variables.

 

The surface area of the cuboid = 2 x (length x width + length x height + width x height)

 

In this formula also there are only two variables in each term.

 

 

Therefore, the unit of area is square units.

 

The volume of the cube is V = (side)³      

 

The volume of the cuboid is V = length x width x height

 

In both the formulae, each term is having three variables.

 

Therefore, the unit of the volume of the cube and cuboid is cubic units.