Why 'equal' in sentence: All men are created equal?

I read a sentence ’ All men are created equal.’

On the surface, I think equally is right but equally is wrong.

‘equal’ is an adjective but why is it in the above sentence?

English is not my first language, but grammatically speaking,
“equal” here is what is called a secondary predicate.

Your sentence virtually means “All men are equal when created.”

The usage is broadly divided into two types:concurrent or resulting.

(1) a. We caught the bear alive (meaning "The bear was alive when we caught it).
b. She pushed the door open (meaning "She pushed the door and as a result it became open).

Both “alive” and “open” are secondary predicates that convey information about the objects (“the bear” and “the door” respectively). “Alive” means the concurrent state while “open” refers to the resulting state.

Seiichi MYOGA

That’s right, Seiichi.

The explanation above is detailed and correct:)

In simple words the meaning is:

All men are created TO BE equal. ie all men are equal, they have the same rights and responsibilities.