Because you want it to.Why does swing sound so "good" ?
Seriously.
'Swing' is one of those popular but really vague words that become so rich with meaning and importance to people that they don't realize that no one ever really defined it very well.
'Punk' is another such word.
As is 'existential'.
I, for one, have always felt that the key to 'real' swing as performed by, say, Duke Ellington's band circa 1940, is to relax while you are playing enthusiastically.
This sounds trivial, but actually there are very few musicians who know how to do this effectively. For example, the more recent 'Duke Ellington' band under Mercer Ellington completely fails (IMHO) to swing in this sense.
All of the verbiage about ratios and whatnot is just an inefficient way of noting this sense of relaxation.
It's like making soup. If you know what you are doing, you never measure anything. In fact, you might even have trouble writing a recipe, because the whole process is variable, based on ones senses of smell and taste.

