I think you mean solar system. While nobody has traveled to Mars, it is reasonable to suggest that is only because we haven't tried hard enough yet. There are problems in the way, but they seem like engineering things that we will figure out if we try. It is debatable if we can make a self supporting colony on Mars, but it getting there seems perfectly possible in reach. Venus is harder (getting there is easier, but reaching the surface is questionable), but we could probably do it.
Getting just the farthest planet in our solar system though is getting close to a lifetime. The nearest star to earth is 4 light years away, we have no hope of reaching it in a lifetime with any technology we know works (there are nuclear options that seem promising but we don't know if they work)
The milky way is 100k lightyears. Not possible to cross without FTL.
Almost all my favorite books almost exclusively occupy settings with laws of physics that are internally consistent and believable and therefore have no FTL. See "The Expanse", "Three Body", "Blindsight", "The Sunflower Cycle", "Rendezvous with Rama" and too many others to name.
Generally I think the impact of instant communication and near-instant travel (or lack thereof) is not talked about enough. Most sci-fi stories derive from the present day, and keep this aspect of today (as do many fantasy stories, interestingly), but it's a narrative choice with large impacts.