>>508786423 (OP)Reality, for one.
To entertain the notion by taking in its lore, however...
We have not had a period of rule and war by a subspecies of genetically engineered ubermensch, and thus have not had the selection pressures that gave us the descendants of Humanity that populate Star Trek's setting.
I shit you not, the only reason any of the races of Star Trek make it is due to culling events and bottlenecks that force samethink.