Actually Linux and Mac are pretty similar. Someone here can correct me (and educate you further if desired) but from my basic understanding both Linux and Mac are based off from the BSD system.
Not really. They're more similar to each other than they are to Windows, but that's about as far as it goes. Also Windows is getting a Linux compatibility layer, so that comment is only true for native apps.
For starters, Linux isn't BSD based. BSD and Linux both implement the posix APIs and have other similarities, but they're not the same thing.
Secondly, and more importantly, native Mac apps don't use X11, Wayland, KDE, Unity, Gnome, QT, or any of those other familiar linux frameworks. They use Cocoa, which doesn't really have a linux equivalent. So while it's true that filesystem, networking, and opengl techniques are nearly identical between Mac OS X and Linux, the GUI layer is entirely different.
Finally, don't underestimate the impact that small differences can have on development costs. Something as simple as the fact that Linux tends to use a case-sensitive filesystem, whereas MacOS and Windows do not, can require substantial development and testing effort to work around.
chris