Because who’s to say in this world that Barry Allen or Wally West – Barry Allen if it’s Grant – isn’t a courier? He doesn’t have any powers, necessarily.
After all, if everyone has powers and advanced technology at their fingertips, than heroes would not be all that special.
We can’t rule out that Grant Gustin could show up some other time playing somebody else, that there’s a little bit of a crossover.
And sure, there’s plenty of normal humans in the lives of the heroes.