The Q, as seen, can create an universe/a reality by snapping his fingers - omnipotent/beyond physical law, etc.
All the forerunner achievements you named are child's play by comparison - and certify that the forerunners were most definitely bound by physical law.
Not to mention, none of these ancient "god-like" civilizations were powerful/smart enough to prevent their own extinction. Which is more than enough to destroy any "god-like" mystique.
That's why I wanted to keep "Q" out of this discussion. Heh. We already know him and Q's are grossly powerful. I think I probably wanted to keep more on track with a civilization that started off mortal/corporal and eventually climbed its way to the top. Q were all powerful beings since the beginning, as far as we know.
BTW Edit, yeah the Forerunners got pretty hurt badly, one might say but then there's the Precursors. They are considered Transentient. Nobody knows what happened with them but guesses are they left the galaxy and/or this universe. All we know about them is that they had technology that worked on "neural physics", seeing as how they believed the universe was a living thing and that at least one of them might have become a Gravemind (The Primordial/Timeless One).