The point is that the USA is still involved in wars and has been for over 90% of it's history.
Only using a very loosey-goosey definition of "war" that seems to be anytime a US bullet is shot in a country counts as a declaration of war. Using such a broad-brush lowest-common-demonimator definition, very few countries are going to have much better stats than the US in this regard. If for no other reason than we typically only get involved in foreign conflicts as part of an international coalition. So if Kosovo counts against us, it counts against all of NATO as well, which renders specific criticism of the US moot.
Under the US constitution, only Congress can declare war and that has only happened five times in history (1812, Mexican, Spanish, WW1, WW2). Using the more common "blood and treasure" standard that would make an unofficial action more apt to be called a "war", there's:
Revolutionary War (7 or 8 years depending on what is considered the starting point)
War of 1812 (3 years)
Mexican War (2 years)
Civil War (4 years)
Spanish War (3 months)
WW1 (1.5 years US involvement)
WW2 (3.5 years US involvement)
Korean War (3 years)
Vietnam War (depends how you count it. 19 years max)
Iraq 1 (7 months)
Afghanistan (ongoing, 15 years so far)
Iraq 2 (8 years)
So totalling that up gives 67 years roughly, but some of those were concurrent, so knock off Iraq 2 since all it's years happened during Afghanistan so that makes it 59 years at war if we give Vietnam a full 20 and start the Revolution based of the British counting of 8 years. So 59 years at war divided by 241 years since the Declaration of Independence gives us a percentage of 25% of our time being at war.
If we only count from the ratification of the Constitution in 1788, then we get just over 22% of our time spent at war (not counting the Revolution because it predated the Constitution).