You underestimate the difficulty of compiling from source for the layman. OSX and Windows do not come with compilers out of the box. The user would have to install them and the proper environment for them first. (cygwin, etc...) Then they'd have to get and install the dependencies properly. Finally they'd have to figure out the proper incantations to make it compile, which if you've never done it before can be extremely daunting. How on earth was I supposed to know what an LPATH is or pkgconfig? Not to mention the extremely verbose and scary looking errors from gcc.

That's assuming that it's a program written in C/C++. If it's another language then you might have another huge hurdle to go through. (Properly installing Go in my PATH took me a while). That's also assuming they don't rely on IDE-specific make files. A lot of the time OSX applications rely on xcode project files. The other day I was trying to compile an Android app before realizing that it was written in a language by Adobe which required the purchase of a full suite.

Open source can be really limiting if the developer doesn't make an effort to document the build process well and doesn't use open tools to create the product.