Because a concept is vague enough to continue the "war" for any reason and concepts don't require congressional authorization to wage war against them. Granted, actual countries don't seem to need said authorization these days either.
War on Poverty= projects are a mess, HUD is a disaster, our welfare programs cost more than other nations' similar programs, urban blacks are still stuck in the same cycles of violence and poverty they were in the 60's when the "war" began.
War on Drugs= Billions of dollars blown, militarized police, massive intervention and manipulation of foreign governments, huge profits for criminal organizations, outright collusion with criminal organizations in some cases, wider surveillance nets, dramatically increased outright physical violations of the 4th amendment due to "probable cause", insta-warrants pioneered during this "war", gigantic and largely redundant government agency infrastructure added, and booming prison populations full of nonviolent potentially tax-paying citizens.
War on Terror= even further violations of 4th amendment, practical panopticon of surveillance, extrajudicial holding of citizens, extraordinary rendition, approved killing of US citizens, violations of the 1947 national security act by the CIA operating within the US, the use of torture, secret military hearings for US citizens, and an amount of money spent that makes the "War on drugs" spending seem frugal by comparison.