I definitely agree that there should be a little overlap when it comes to the skills and responsibilities of QA and Sec, but I think youre making an oversimplification of what we do.
Im not very fond of using automated scanners myself but something you program in 20 minutes is not going to have the same breadth or depth of the test cases it performs. There are thousands of ways to avoid certain defensive methods and plently of special case scenarios that require this bulk testing. I agree creating tools or test cases should be within the skillset of a security professional, but I think youre making it sound like if something is vulnerable to sqli or xss itll pop after 5 attempts...
In my experience automated testing does a lot to give you good places to look or identify patterns about the application, but nothing beats good ol manual analysis