The simple fact is that the developer has decided that they wish to include combat in their game, so whether or not other adult games do this is not really relevant.
Not all engines come with combat mechanics built-in so that influences the possible choice of engine, as would any other personal requirements the developer has. (like their level of programming knowledge)
People play individual adult games for any number of reasons, so I won't presume answer for them, although the fact that there are people that seem to like playing this game seems to indicate that some people do. One thing I do find amusing about your combat related complaint is the fact that don't have to actually participate in any of the combat because you can sneak past it all.
