Why is it considered bad to wash relaxed/chemically straightened hair everyday?

I have thick curly hair that I relax and color 3x/year, I have done so for the past 4 years. I wash my hair 4-7 times a week and deep conditon once a month, and my hair is to my mid back and healthy. I work out everyday and I love to sweat, so I have to wash the sweat out or else my hair gets greasy and smells bad. Why do so many people tell me that relaxed/chemically straightened hair only needs to be washed once a week? I tried co-washing and my hair looked dull and waxy, so I use shampoo.