Usually because it's not necessary: as you have probably noticed, the beam from an LED light tends to have a combination of both - a bright central hotspot surrounded by a much wider spillbeam.
The small size of the light emitting region in LEDs means that you can get very smooth beam profiles with few artifacts - effectively you have a point source which can be placed in the correct position relative to the reflector.
If you still have an old M@gLite you might want to compare the beam to that of your M20 - in addition to it seeming very faint and yellow you'll probably be shocked at how non-uniform the M@glite beam is. Horrible, ringy and probably with a great big doughnut hole in the middle. This is an inevitable consequence of having an extended source (i.e. the filament in the bulb): not all of the light emitting region can be at the focal point of the reflector.
The variable focus of these older lights was in part a way of covering up the horrible beam quality - at least you could adjust it to get a more or less worse effect. With LEDs this is no longer necessary.