Here is why humans aren't allowed to leave......at least not by the aid of the benevolent races.

ALPHA-DRACONIANS
Reptilian beings who are said to have established colonies in Alpha Draconis. Like all reptilians, these claim to have originated on Terra thousands of years ago, a fact that they use to 'justify' their attempt to re-take the earth for their own. They are apparently a major part of a planned 'invasion' which is eventually turning from covert infiltration mode to overt invasion mode as the "window of opportunity" (the time span before International human society becomes an interplanetary and interstellar power) slowly begins to close. They are attempting to keep the "window" open by suppressing advanced technology from the masses, which would lead to eventual Terran colonization of other planets by Earth and an eventual solution to the population, pollution, food and other environmental problems. Being that Terrans have an inbred "warrior" instinct the Draconians DO NOT want them/us to attain interstellar capabilities and therefore become a threat to their imperialistic agendas (Draconian).