It wasn't always like this. There was a time when men's clothes emphasized broad shoulders, narrow hips, and long legs. British military uniforms in particular were the human equivalent of peacock plumage. Witness Major George Cunningham here, circa 1810 or so:
See what I mean - the trouser stripe highlights the length and straightness of the leg, the diagonal embroidery on the lower rib cage points up the width of the chest, the pointed bottom hem and belt of the tunic emphasize the wasp waist and lead the eye downward to where the embroidery on the pants flank the, er, naughty bits. (The horse is handsome too.)
A larger version of this image may be found at https://www.pinterest.com/pin/199706564699213302/. It's on Pinterest thanks to 2 Nerdy History Girls, whose joint blog is at http://twonerdyhistorygirls.blogspot.com/. Their Pinterest page is at https://www.pinterest.com/2nerdyhistgirls/.
This clothing didn't just say, "I am a male," it said, "I am a fit, strong, healthy male. Fight me if you dare." The look worked for mating as well as war.
I have a theory. My theory is...
Men's clothing today is so drab because powerful men don't want young whippersnappers to upstage them in the attractiveness department. Back in the Gilded Age, filthy-rich guys like J. P. Morgan were the model to whom ambitious young men aspired. Morgan was also one ugly dude.
Of course there's no accounting for tastes - those who favor men who resemble a cross-eyed walrus with a paunch would likely fall in love with him.
In the 19th and early 20th centuries, physical fitness was not considered a sign of virtue and hipness, as it is now. Consciously or not, captains of industry saw the advantage of concealing their short, portly, bandy-legged bodies with loose draperies in dark colors. So now all men who want to be taken seriously in the business world have to dress like short, portly, bandy-legged robber baron capitalists.
Rise up, guys, and show off the physiques you've worked so hard to build! Make haberdashers create clothes that make you look like men again! (But please, no codpieces, those are just vulgar.) You have nothing to lose but your uniformity.