curiosity had to deal with many firsts, like the sharp rocks puncturing its wheels at a higher than predicted rates you want to have many of these 'firsts' mom

curiosity had to deal with many firsts, like the sharp rocks puncturing its wheels at a higher than predicted rates you want to have many of these 'firsts' moments before launching humans, as long as there's 'firsts' it's going to involve too much risk, even if you plan not returning the humans, getting some there requires a couple order magnitude infrastructure as a rover, so you don't want to lose your investment day 2 from some unforeseen accident. of course you can't predict everything. but the fact that we're still discovering this many issues about the environment is a good indication we haven't discovered enough to get risk to a manageable level