While dinosaurs were living almost everywhere for millions of years, human giants lived roughly for few centuries in some isolated area.
Its very natural that while we have been discovering one dinosaur skeleton after another, from China, through Russia to Africa or America, human giant skeletons can be very difficult to ever find.
Logically.
Also, to say "a giant" can mean many things. If people were much smaller than today, a giant could be a 2,5 meter man. Or, a giant can be name for very powerful people/kings/beings, half-gods.