Portability is what made UNIX the most widespread and successful operating system of all time.
As an engineer yes I want to solve practical problems with a computer, but I also don’t want to dictate (within reason) which OS the user must use. For example, if one of potential users of my software has a really advanced FreeBSD or OpenBSD setup and they can compile link and package my software, it would enable them to keep the advantages of their setup, and my software would make their advanced setup even more useful, like compound return on investment. Another advantage of this approach is that by not dictating the platform, it makes the user more productive and saves their time. People just want to get a task done and solve a problem, and respecting their time should be one of the programmer’s priorities.
We're not dictating what the user is using for their OS, again as I mentioned, there should always be a pure POSIX fallback. But #ifdefs help to take advantage of what the OS can do if the user decides to do it and we should not take that away from the user either.
Additionally I feel like it should be mentioned that even when POSIX and UNIX was a thing, programs had to include #ifdefs because target platforms wouldn't support X or didn't support Y in the same way as another platform. POSIX may have been designed to help against that but people still had to port their software a lot, with lots of #ifdefs and runtime shims.
As an engineer yes I want to solve practical problems with a computer, but I also don’t want to dictate (within reason) which OS the user must use. For example, if one of potential users of my software has a really advanced FreeBSD or OpenBSD setup and they can compile link and package my software, it would enable them to keep the advantages of their setup, and my software would make their advanced setup even more useful, like compound return on investment. Another advantage of this approach is that by not dictating the platform, it makes the user more productive and saves their time. People just want to get a task done and solve a problem, and respecting their time should be one of the programmer’s priorities.