> Why in the design of Windows aren't programs installed or symlinked in the PATH by default? I guess that was a design choice somewhere along the history of Windows/DOS. Is there a reason?

Windows' way of program executable placement is using the holy Registry. It's called 'Application Registration'[1] and was introduced to reduce the needs to modify the system-wide PATH variable. (They thought it was a bad idea to modify a system variable so frequently, and I partially agree.)

You can find registered applications in `HKLM\Software\Microsoft\Windows\CurrentVersion\App Paths`. Very few programs use that feature, which is unfortunate, but popular applications like Chrome and Firefox register themselves in it. That's why you can invoke `chrome` in the 'Run' dialog.

Edit: Another context to add: at the time App Paths was added, to modify PATH you had to edit AUTOEXEC.BAT manually which was painful. Not only that, but also PATH had a length limitation of 128 characters. You can find more details in the Raymond Chen's blog, as useful as always.[2]

[1] https://msdn.microsoft.com/en-us/library/windows/desktop/ee8...

[2] https://blogs.msdn.microsoft.com/oldnewthing/20110725-00/?p=...