The fae, when creating the many races of the web, were often more interested in the different magics they could give each of them and so many were based on the human race. In part this was because many of the fae who had survived the end of the magic on their old world could be seen as the lesser fae, because they were more humanoid. Only a few of the greater fae, the less humanoid, managed to survive. Some had a part in making sure that there were at least a few races that were based on those who had lost their lives. No one is quite sure why, although some believe that the greater fae needed more magic in order to simply survive, while others think it is due to the stronger connection the greater fae had to their old world.
As the world creators worked together with the race creators, and there was often several of them working on each world, some worlds were created specifically for the races that were going to live there while others were created in order to test the races who would be placed on that world. Everything they did was an experiment, because most of them didn't care much about who they were creating or what would happen to them later on. It wasn't because the fae are uncaring in general, but the Yellow family is still seen as the least caring of the thirteen families, more interested in what they can do with their magic than anything else. Some of the Yellows really are like that. Others did care about what they were doing, but they were entirely sure that what they were doing was the only option.
Due to their lack of understanding the fae's experiments could easily have gone very wrong. Almost all of the races were given some form of magic, often based on a magic the fae knew of, although they were just as experimental as everything else they did, and this could have ended everything. Instead they got lucky. They had landed on a world with some understanding of what needed to be done and it worked on making sure that those things were done, along with its newly created child worlds. Unfortunately the fae don't know this, so they believe that everything they did worked, apart from those things they know for certain went wrong (like the doors leading to Taithmarin, Kankirin, and Aerith).