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A large contributor to the "feel" of an Operating System comes from the software it chooses to include by default. There are entire linux distributions that differentiate themselves just based on the default configured software. There is room for so many options of base software because there are in general many different options to pick. Linux being just a kernel itself specifically creates an environment which encourages this (to some extent). This is both a blessing and a curse, for people wanting to write software targeting linux there now is some matrix of options they must test under if they want it to work with all the various software choices. BSD systems, unlike Linux, tend to include more than just a kernel by default, generally including some "blessed" c library, c compiler, libraries and programs. This makes targeting the system a bit easier, in the sense that you can assume there is a larger set of software on a BSD machine than you could with some system that uses a Linux kernel. Even with BSD however, not all of the software is in control of those in charge of the system itself, there is still a large amount of shared code between these systems for things like the c compiler. There are some pros and cons to this situation. On the plus side these systems get to reap the benefits of the work put in to these other projects (gnu, llvm, and so on), but this also does lead to there being some differences in design. Put in another way, if someone wanted the ability to touch every line of code (in the upstream sense), they would have to be a member of some non trivial amount of communities.