In fact, tens of millions of people frolic everyday in environments far more deadly than space - oceans, lakes, pools, and bathtubs. The oceans and fresh water bodies also offer something space doesn't - various ravenous and poisonous beasties such as sharks, crocodiles, and sea snakes.
Space is simply different.
Magnifying that difference is the fact few regular people have been into space and certainly not for prolonged periods of time. The main physiological difference is that passengers experience a sense of weightlessness. Being in prolonged freefall does have certain effects on the human body, but dealing with these effects are really no different than dealing with the effects that divers must take into account when they stay at depth for any length of time. We deal with living, working, and playing in potentially lethal environments in our everyday lives. The space experience will be no different.