There are a few reasons why we haven’t sent a manned flight to a planet like Mars yet. The distance is one. At their closest Mars and Earth are still 80.5 million kilometres (50 million miles) away, or about seven months. They’ll need supplies on board for all that time.
Even if advances in rocketry can reduce the time, once on Mars astronauts will have to wait up to a year and a half to return, when the orbits of the planets align just so. A spaceship big enough to contain all the necessary supplies would be too big to launch from Earth’s gravity given our current technology, so we could theoretically build on the Moon or in orbit.
But launching parts up over time like that could get expensive. Beyond these issues, astronauts going to Mars would be exposed to high levels of radiation as well as suffer the effects of microgravity (losing about one per cent of their bone mass each month). There would be psychological fallout from being so far from home too.