Why do they make sleeping bag around 7 1/2 foot long when the average person is only 5' foot 7"?

sleeping bags usually run 70 to 78 inches long ... but remember, that's external measurement, not internal.  Loft is what keeps you warm.  Depending on the rating of the bag, you should expect 2"+ of loft, which means the bag isn't as long on the inside as it is on the outside.  They make different length bags for different sized people.  A 7 1/2 foot bag is for people over 6' tall (like me).  I'm 6'2" tall.  Which means, if I want a 20F bag, there's going to be about 2.5" - 3" of loft at the head and foot ... that's 5 o 6 inches of length gone from the bag.  That means it will just fit me (74" tall + 5" of loft = 79" of total length used).  That's why.

My preference is a down quilt (mine are all from Hammock Gear, but UGQ, Enlightened Equipment, Loco Libre and Jacks R Better all make great stuff).  There's no hood, and I still use a 78" quilt, because it gives me the right amount of coverage and warmth.  When it comes to insulation, you don't want to carry more weight than required, but you also don't want to be cold.

Because us shorties can sleep in a bag that's too long a lot easier than a tall person can sleep in a bag that's too short. 

There are people over the average height. I'm one of them.  I need over 6' just to fit.  

And, if you put one foot in boiling water and the other foot in ice, on the average, you'd be comfortable.

Because they come in different sizes so you bought the the wrong size.