Think about your profession as a tester. Do you have control over the source code? Do you control the project budget or the timeline? Do you make decisions about resourcing? Or are you in charge of the specifications and contracts?
For most testers the answer is no. All of the above-mentioned are aspects of quality, yet a testing professional doesn’t have any control over them.
The only thing that you have as a tester is the power to influence the people who have it.
And that is why I call testing a soft skill.