At risk of crassness - human lives are pretty cheap and there are plenty of people willing to take the hit for a chance to be in space for an extended timeframe. Meanwhile building something with enough spin and shielding is a huge ask
We don't even know how much spin we'd need, and this is an important question to answer if lunar or martian habitats are something we're serious about. Maybe enough spin to match lunar gravity is enough, maybe less, maybe a lot more.
If manned stations aren't doing any particularly unique research, especially research that couldn't be done with automation, why spend huge resources on them?