Who made the Room of Requirement?
Rowena, the academic who wanted to teach others how to help themselves with books to teach them themselves?
Salazar, the inward thinker that likely enjoyed time to himself who wanted to teach others of all that magic can do, whether it was light, dark, or otherwise?
Helga, the friendliest person amongst them, that wanted to teach all to help others as they help themselves?
Godric, the chivalrous, that wanted to teach others to do what they can with what they have?
All four of them had extremely good reasons to build the RoR.
All four of them came together to build a school, and multiple buildings to help others help themselves and others.
Even so, I believe the RoR was built long before Hogwarts. By Merlin. For Arthur Pendragon.
Think about it. A room to train, and think, and live and be one's self. It screams out as a perfect gift to Arthur. Yes, the RoR is on an upper floor, but that's why it's magic. The four founders probably didn't even know it was there since it was covered in an extremely thick blanket of magic created by the most powerful wizard ever who was long since dead and built Hogwarts around it unwittingly. Besides, the four founders all had their own special crevices. The Grand Library for Ravenclaw, the Chamber of Secrets for Slytherin, the Great Hall for Hufflepuff, and the Quittitch Field for Gryffindor.
A place to read for an academic.
A secret hideaway for an indepent thinker.
A place to talk and chat for the most friendly person possible.
And a place likely first used to create something out of nothing for a person who chose to live life as free ride. (On a broom)
They all had reason to build the RoR, but had projects of their own.
