Space habitats are supposed to spin
The ISS was designed strictly as a micro-gravity research laboratory. Out of the countless design variations of Space Station Freedom and the International Space Station, one of the dominating criteria was protection the laboratory modules from acceleration, tidal forces, vibrations, and even crew movement. Quite a few cost-cutting designs were tossed because they didn't protect the lab modules' microgravity conditions. For example, the simple gravity-gradient stabilized Power Tower concept for Freedom was dropped because by using lab modules as ballast at one end of the tower they were subject to some tiny G-forces.
You don't get a microgravity environment by spinning the entire space station. You don't get a clean microgravity environment in a lab if part of the station is spinning, leaving the rest of the station shaking, shivering, and suffering gyroscopic precession as it orbits Earth.
goin up there to lose muscle and bone mass is the most poorly thought out thing that ever came out of the 60's and it's amazing that we haven't figured that out yet.
The ISS's research work is dominated by two fields: biology and materials science. A great many of the biology studies coming out of the ISS, like the Twins Study, are all about that bone and muscle loss. The whole point of 6-month and 1-year tours on the ISS is to find out what spaceflight does to humans because the 1960s left a lot of blanks.
The 1960s and 1970s 3-year Martian roundtrips or 14-month Venus flybys envisaged at the time would've crippled and incapacitated the astronauts and cosmonauts because space agencies of the era were clueless about long-term spaceflight effects.
The ISS's non-spinning environment has answered a lot of important questions over the last two decades of service, questions that would not have been answered with a spinning space station.