This is just my guess, but I think that when you work for a company, where once everything is compiled (converted to binary data, which computers can execute), nobody can see the source code anymore, you tend to be a lot lazier, as it doesn't matter as long as it works.

Linux is open source, so everyone can see the source code if they would wish to do so. If you are lazy, and write a really crappy piece of code which will really slow down the system or something of the sorts, there goes your reputation, as everyone just saw. Developers and users alike. I think that's one of the reasons why open souyce developers tend to be less lazy.

It's also a fact that the core of the Windows kernel was never written with the intention of becoming what it is today (neither was Linux', but they did the effort to optimize the old bits), and developers have simply always been adding and adding on top of it, until the code was so cryptic that nobody could make something out of it anymore. Two properties of messy code is that there's often a lot of redundancy, and that you don't see the links between two pieces of code so easily anymore, which makes that you can't even try to start optimizing it. Just an example (not a cryptic one);

for (int i = 0; i < 20; i++)

{

    if (x == y)

    {

        if (z <= u)

        {

            goto banana;

        }

    }

}

The example above could be done much more efficiently;

for (int i = 0; (i < 20) && (x == y) && (z <= u); i++)

{

    banana ();

}