No excuse, other than not presently pursuing those forms of sexual gratification. To some men it is great, to others it is not.
Kind of like asking a group of men, why do you not try topping or bottoming, some may say sure, some will say no way. Then you ask them why, what is your excuse ? They may answer , just my choice
Heck you could even go as far as to say, not every Straight man wants a BJ, from every woman he comes into contact with, even though they all have mouths, Hell, not every GAy - Bi man will accept or perform oral on every man they come in contact with, Ask any of them why; and they may answer I do not want to; or just not my type.
All men may be created equally; but what they do behind closed doors often greatly differs . Most often with out rhyme / reason ; or / nor Excuse.