Because usually in the past, the big leap was not done by playing it safe, but by ignoring all the naysayers, disbelievers and those without imagination and doggedly doing the impossible only to discover that when push came to shove, solutions were found to previously impossible problems and when enterprising people put their heads together and do not consider failure an option, miracles can and do happen.
Playing it safe can net small incremental improvements, but it won't capture people's imagination and it won't energize the human spirit to reach to the very bottom of our abilities. Playing it safe won't unite a large group of fundamentally egoistic beings for a common cause.
I believe the biggest evolutionary leap that propelled humans to the top of the food chain is the enterprising human spirit, the pioneering drive, the irresistible urge to stick our fingers into the wallsockets of the universe. Sometimes we get shocked, and sometimes it hurts badly, but we strive becase we do not give up. We reach for the stars, and when we find ourselves in dire need because the cold, merciless and deadly vastness of space threatens our survival, we are pushed to find solutions that were deemed unthinkable otherwise. Perhaps the first shot only gets us to the moon, but we'd never reach it just plinking arrows. Early explorers set out into the unknown oceans knowing that they will likely not return. Many faced extreme hardship and regrettably died. But some survived and they wrote history as we know it. They were all ill-prepared, because they didn't even know what to prepare for. But it is their sacrifice that allowed us to discover new continents, to circumnavigate the world and connect pockets of humanity into an ever denser network of modern society.
There was a time when scientists were convinced that humans will not be able to breathe if travelling faster than 40 mph. There was a time when human flight was thought impossible. It took a bunch of people jumping off cliffs and plunging to their death to figure out the details. Those jumps were necessary. Not just for lessons learned ("this is not the way to do it" is a valuable lesson!) but each of those jumps pushed the envelope a bit further. Each pioneer got a bit closer to the light at the end of the tunnel. The light in some tunnels ends up beign a train, but some tunnels end up leading to the bright spark of revolution.
As much as logic dictates otherwise, we need dreamers who are irrational and fearless enough to go into the unkown. Without these dreamers, nobody will push the envelope. Most of them will be considered foolish, stupid and reckless in their time, but our history is built on the shoulders of those who succeeded against all odds.