I do exactly the same thing whenever I write C/C++. Genuinely large projects aside, I see no compelling reason to waste your time with build systems. This approach is simple, easy to maintain and lets you get on with writing actual code.
Because I don't see any value from using it. It's not easier to read or write, it doesn't force you to keep it sane and simple like a bash/cmd script generally would, and it's just another dependency I don't really need. If I take a minimalistic approach to build systems, I prefer to go all the way.
Also, although this is merely a personal quirk that shouldn't persuade anyone else, I've seen enough horrific, unreadable make files to instinctively dislike them by now.
In my mind, shell scripts are simpler. I have no reason to need the extra complexity that make brings.
Also, I do a lot of programming on Windows, where GNU make would be another dependency to install. (Also, in my experience, make is slow on Windows, since they have to emulate fork()). I guess I could use Microsoft nmake, since I assume it's still installed along with Visual Studio, but again, batch files are simpler.
And for GNU tools on Windows, I would heavily recommend MSYS2 these days - having Pacman as the package manager is very nice, and there are already a lot of packages there.
