The question should have been why resistance increase if length is increased but not increased when area of cross section is increased?

Answer:

The flow of electrons results in electric current.  The resistance is inversely proportional to current flow. The resistance of a given material is given by R = Ρl/A where l is the length and A is the area of cross-section of the material.

From the formula, it is clear that the resistance of a given material increases with length, but decreases with increasing cross-sectional area. 

Impact of length of wire:

The electric current is caused by moving electrons in a metal wire. The moving electrons can collide with the ions in the metal. This makes it more difficult for the current to flow, and causes resistance.

The resistance of a long wire is greater than the resistance of a short wire because electrons collide with more ions as they pass through in case of a long wire. The relationship between resistance and wire length is proportional.

Impact of area of cross-section of wire:

Consider a thin wire and a thick wire. In case of thin wire, fewer electrons are there to carry current. So current is less and resistance is more. In case of thick wire, more electrons are present to carry current. Hence, current is more and resistance is low. The resistance is inversely proportional to area of cross-section.