It's rare today to find an application that isn't built on open source. Using open-source components reduces time-to-market. It allows you to focus on what you do well and not worry about what you don't do well. And it lets you take advantage of the skills of many, sometimes thousands, of developers who have built a common component. You'd have to be insane to write all your code yourself.
Not only is more open source code being used, but that code is also more complex, making open-source frameworks and libraries enormous. While all that has enabled developers to offer more fully featured applications, it's also made the software development cycle much more challenging, especially from a security perspective.