If colon cancer usually shows symptoms usually when it's in the late stages. why can't anyone in 30s or early 40s get colonoscopy or ct colonography without severe symptoms?

Some do based on their family history. It has to do with the ability of the medical system, financial and technical, to absorb so many tests with a relatively low meaningful yield. The fact that the age for screening colonoscopy keeps dropping, currently at 45 speaks for increasing incidence at younger ages, likely the result of dysfunctional lifestyle.

If you have family history of early onset colon cancer, you should get a colonoscopy at 10 years younger than the earliest age at which a family member got colon cancer. Testing in younger people has poor yield and the procedure can have complications. Visit this site for info: https://www.cdc.gov/cancer/colorectal/basic_info/screening/index.htm