I have heard that every mass attract another mass with a force directly proportional to the multiplication of their masses and inversely proportional to the square of distance between them, but newton doesn't told that why two masses attract each other. Also I am familiar about the curvature concept considered by Albert Einstein according to which gravity is not a force but a curvature caused by imbalance in spacetime by mass/energy. Bit if there is a curvature how can you prove it?
Physics does not answer "why" questions at the level of basic laws and postulates, i.e. "why" this postulate. Newtons gravitational law :
Newton's law of universal gravitation states that a particle attracts every other particle in the universe using a force that is directly proportional to the product of their masses but also inversely proportional to the square of the distance between them. This is a general physical law derived from empirical observations by what Isaac Newton called induction.
Laws are like axioms in mathematics and are needed so that mathematical proposals fit the data, in this case the mathematical model is the 1/r^2 behavior , and Newton's law is the statement that connects one to one mathematics to data.
So the answer is, because that law is the most economical ( occam's razor) way of describing mathematically the data, and having a theory that can predict new set ups.
In general questions that hit on the postulates of physical theories have the answer: because this is what fits observations and allows to set up a theory that is predictive.