Because it can't happen..
why?
well
1) Light guns are physically made to a type of monitor. So the circuitry will only work on an arcade CRT or a TV or a VGA monitor. You have to double to optics to have more then one option work.
2) The drivers are all different. Main trouble here is nobody is willing to spend the months of coding to write a driver for a few people.
There are some other solutions... non good (yet)
1) Act labs gun system (old version). Has 2 guns.... and doesn't work worth spit. If someone made a version specific for mame it would work well (since it worked well in the demos). But nobody has done it yet.
Also, it doesn't work with EYGO monitors (my problem) or TV's.
There are two guns, but only works with games that have native support.
btw, if you want a gun system, I have one for sale cheap.
2) The best up and coming solution. Actlabs has a new USB gun coming out. This has been tested to work with all mame games (the great part about it).
troubles... VGA only (don't know if it will work with B-Grade monitors yet).
and
only one gun. (1/2 the fun is john woo'n it)
they are planning a TV version to come out soon.
Good points. they are listening to the emulation scene this time... and are even talking about helping people hack their guns into more realistic cases.
3) haven't found a 3rd worth noting. A few companies have in other countries sell guns, but nothing I've heard works.