Why do you say this?
They are for whatever reason. I guess the knee is a more complicated joint. Had my hip done 3+ yrs ago. Was walking steps the next day. I know folks that have had both done and they all say knees are harder.
I had both knees replaced last year, and met lots of fellow patients who had had both surgeries. Hips are simpler than knees. Not nearly as many moving parts. Knees are a marvel of engineering, with many moving parts. The more complicated the part, the trickier the replacement.