Python did not come pre-installed on my Debian distro or (obviously) my Windows 10 PC. When I installed Python using apt-get, it looks like only the basic python distribution was installed. I am working on a program that uses PyGame for its UI, and I had to install pip (to get pygame) separately. So it's not a complete surprise that tkinter was also not installed by default.
On Windows, the installer has checkboxes for certain components. if you don't check the box for pip and tcl/tk, then they won't be installed. (I don't know what the default is, since I already have those on my Windows system.)