Question

Why gravity is still regarded as one of the fundamental forces?

Answer 1

It is a fundamental force in the sense that it cannot be described and explained as being an aspect of any other force.

Explanation:

I'm not sure what you are implying when you include the word "still" in the question, but I'll give a comment just the same.

We describe gravity on the basis of general relativity as being due to the curvature of spacetime caused by distributions of mass. This cannot be obtained from any other force theory (strong, weak or electromagnetic) as a special case or a consequence. So, it must be taken as fundamental.

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