Why current flow in direction opposite to flow of electrons

That is known as "conventional current." You can actually view current as flowing either way; it just depends on how you want to analyze the circuit.

When Benjamin Franklin was theorising about the nature of an electric current he thought that it was some sort of 'fluid' that flowed from an area of high pressure, which he labelled as 'positive', to an area of low pressure, which he labelled as 'negative'. Although we know that, in metal conductor at least, an electric current is a flow of negative charges (electrons) that flow from negative to positive which is nothing but more commonly known 'Conventional Flow'.

Originally the true nature of electric current was not known. It was assumed to be positive charge moving. Later the electron was found to be the primary carrier of electric current in metals, so electron flow was considered. YOu can use either convention in modeling and calculating circuits as long as you use the proper polarities on components.
In some semi-conductors (things that do not conduct as well as metals) it can be shown that the primary thing carrying current is indeed a positive charge, but this gets tricky to explain, It takes what is called the Hall effect.
Hope this helps.

In a welding arc or in battery electrolyte, metal ions actually move in the opposite direction from the electrons, so early scientists thought that was the flow direction.

Electrons only flow in DC and - apparently - very slowly.

What actually flows is energy.

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