I'm not entirely certain on what metric you are using to say Windows is more secure because it has had more eyeballs. I would assume the fair metric is eyeball hours per line of code, which I would assume is much lower for Windows than for Linux. There are probably many parts of the Windows codebase that, once written and demonstrated to be working, have probably not received any real attention since. With Linux and other popular open-source programs, their lines of code have probably been reviewed by every serious senior software engineer that have decided to get involved in those codebases for one reason or another.
There is no reason to think that the eyeballs looking at the Windows source code (i.e. Microsoft employees) are any more skilled than those in the open source community looking over Linux source code. There are probably many parts of the Windows codebase that was written by junior engineers, given the OK by a senior engineer if the code worked and didn't look obviously broken. Much of that code probably hasn't been combed over since because it would only be combed over by the few people that have access to it and the fewer still number of people who have both access and a reason to comb over it because it's part of a task they are currently assigned to work on.
There is no reason to think that the eyeballs looking at the Windows source code (i.e. Microsoft employees) are any more skilled than those in the open source community looking over Linux source code. There are probably many parts of the Windows codebase that was written by junior engineers, given the OK by a senior engineer if the code worked and didn't look obviously broken. Much of that code probably hasn't been combed over since because it would only be combed over by the few people that have access to it and the fewer still number of people who have both access and a reason to comb over it because it's part of a task they are currently assigned to work on.