You can move a big pile of dirt with a shovel, but if you have a backhoe handy, it’ll go much faster. Making software more secure works the same way. You can hunt down vulnerabilities manually, but if you can use automated tools, you can accomplish so much more.
A single software application could have many thousands of lines of code, sometimes millions, especially if you add in the open source components that are part of the application. It is simply not feasible to have developers hunt security vulnerabilities manually.
Security tools are not as smart as humans (yet), but they can do a huge amount of analysis in a relatively short amount of time. Effective security programs use the best parts of tools and humans; tools can cover broad areas quickly, and humans can do more targeted analysis.
Some activities simply cannot be automated. Most, if not all, the security analysis that happens during the design phase, for example, must be done by humans. But during development, testing, deployment, and maintenance, a variety of useful automated testing techniques help flush out security vulnerabilities.