first, nothing can really make one's penis bigger after his 20 years old, so give up that part.
libido is an obvious thing and can be felt quickly, compare to muscle growth. the glands can turn big or small much quicker than muscle too.
When people taking something, and the libido changes, they would be exciting and want to tell. But when talking about muscle gain... you need to wait for at least a month, and it's also about your training, your food, etc...
Another reason is, why people want to be muscular?
Some people feel that it makes them more like a man, more masculine. Then being always horny and good performance in bed are also in that 'alpha-male-like' mental area.
Not to mention some people want to get muscular and leaner because they want to look better, at least partially for better chance on dating or sex.