Why gravity considered as acceleration?

Answer 1

This is a pretty deep question, and it is what prompted Albert Einstein to formulate his theory of general relativity.

The basic is the so-called equivalence principle, that acceleration and gravity are the same. Einstein became to understand that there is no local way to distinguish gravity from uniform acceleration.

A thought experiment would be you inside a closed up room. Without any way to look or detect anything outside the room you will not be able to distinguish whether the room is inside a gravitational field or uniformly accelerating. No experiment that can be carried out locally can make the distinction either.

As such gravity and acceleration has to be the same. In fact acceleration inherits all the hallmarks from gravity, including gravitational time dilatation!