Well that's true, but everything can be decompiled at a certain level.
The roms are only like 1 k a file. Around 5-10 "pages" each.
Even though it's compiled you can sorta see the code.
First rom genrally has some actual code at the top (I'm assuming for the scoring, hud and splotch animations) and the rest appears to be frame/targeting data. And then on soem of them there is a second rom file that has additional targeting data.
Of course I'm just guessing here, it's already assembled so a person really can't tell.
Even digital leisure and the new company that makes the six gun select cabs appear to have written their own code from scratch so I suppose it is harder than it looks.