why do games lock at 60 or 30 fps, why not inbetwen? 40fps? 50fps? is there something am overlooking?

why do you want to do less than 60? Do you not use time_step to make your game fairly framerate-independent?

I think the only reason for frame rates of 30 or 60 fps is the american analog television system NTSC (approximately 29.97 FPS).
Perhaps most game developers thought that if 30 FPS is fast enough for television, it also must be fast enough for video games. Later when the graphic cards became faster they doubled the frame rate to 60 FPS.

Because your graphics card will live longer and consumes less energy when you lock your framerate. Why do you want to torture your graphics card with for example 120 FPS, even though 40 FPS is fast enough for your game?!
The framerate you use for your game depends on the type of your game. Only if you use very fast movements in your game (for example in racing games) you should use high frame rates.

In fullscreen mode, the frame rate is kept in sync with your monitor's refresh rate. This is usually 60. If it can't manage 60, it jumps to an integer division of the refresh rate (60/2 = 30, 60/3 = 20, and so on).

And if the monitor can manage 60 fps, it is not possible to becomes 50 fps in FULLSCREEN. Only in window mode are all fps possible without lock.

In fullscreen mode, DirectX keeps the frame rate always in sync with the monitor frequency between 60 and 80 Hz in order to avoid tearing artifacts. The screen refresh is artificially delayed for matching the time when the monitor has finished its video cycle. This delay is visible in the 'screen refresh' time in the [F11] panel. Thus you'll never get a higher frame rate than your monitor can display. Your frame rate will be an integer division of the monitor frequency (such as 60, 30, 20, or 15 Hz when the monitor frequency is 60 Hz). When the frame rate is close to the monitor frequency, a small change of the rendering time will cause the frame rate to suddenly jump from 60 to 30 fps or vice versa.