Large companies have built in processes for security, which slows down their software development, but also ensures better security than someone hacking together a site from scratch as quickly as possible.

Most of us hackers are jack of all trades, focusing on web design, databases, business logic, and security. It is difficult to be an expert in all areas while simultaneously focusing on speed of release. Just because a company has security experts doesn't guarantee secure software (Microsoft), but in many cases it does help.