Why do humans circulate monosaccharides instead of disaccharides as in plants?
One of the reasons could be  -


  humans and many other animals have active circulatory systems,
  there is little need for targeting sugars for specific parts of the
  body. The entire body would rapidly reach equilibrium with the glucose
  level of the bloodstream, and thus there likely never evolved the need
  to target specific organs to receive sugars via disaccharides


Source : This

Other reason could be that disaccharides are harder to transfer inside the cells than monosaccharides.