A lot of guys do not say out loud that they are already in committed relationships..Several reasons for this may include the fact that they do not want to be viewed as "off the market", in case there are potentials around..
There might be some guys who do not say it because they prefer to keep their personal lives for themselves..However, I think that when someone asks a guy if he is taken or not, the guy have every responsibility to his gf to be honest about it..