Although co-washing is beneficial and popular around here, you can keep your scalp pretty clean by simply putting less product in your hair and on your scalp. I like to use shampoo with every wash, or else my scalp doesn't feel clean at all.
For some, the manipulation of daily washing/co-washing can be counter-productive. There are ladies here who only wash 1-2 times a week (like me), and some that go as long as 2 weeks-1 month.
As far as the differences between other races, I think it's our improper use of relaxers & other chems; naturally drier, coarser, kinkier texture; and (slightly) slower growth rate that makes the difference. Using heat and not properly protecting/moisturizing our ends affects retention. For many years, our hair care practices were modeled after the mainstream and we are finally learning what works for textured hair.
With all that said..... last summer I spent a lot of time co-washing and bunning and my hair loved it.