There are already closed off habitats made to simulate living without outside assistance. It's the combination of a lot of things that are relevant here includi

There are already closed off habitats made to simulate living without outside assistance. It's the combination of a lot of things that are relevant here including zero-g not just any single one of them. We haven't made space travel safe enough yet, we haven't made enough progress on habitats in space. Plenty of things to figure out and a space station is by any means a much more effective way to do that both financially, when it comes to speed of iteration and ability to course correct and focus on improvements.