You have to bring back so much stuff with you in order to survive, not just clothes appropriate to the period and gadgets to dazzle the natives with but oxygen if you plan on breathing, a habitable planet to walk about on as this present one will have moved on elsewhere, even a few hours back in time could spin you half a continent away or, more precisely, to some other part of space completely as the stars, the galaxies keep speeding, leaving you behind. Much easier to stay where you are, let yourself imagine you've just arrived here, a being with limitless powers, miraculously hurled from the future, about to change the world.