The perimeter is a length. Length is measured in terms of appropriate units like meters, inches, etc.
Area is a product of 2 lengths. For example the area of a rectangle is the product of the length of its sides, the area of a triangle is also the product of two lengths which in this case are the length of the base and the length of the height. As a result of this area is expressed in square units, like square meters, square inches, etc.
Volume is a product of 3 lengths. For example the volume of a cuboid with sides w, l and h is w*l*h, the volume of a sphere is (4/3)*pi*r^3, etc. As a result volume is measured in cubic units, like cubic meters, cubic inches, etc.
because volume measuers three things