why does iPhone 6 plus 64g only come with 55.7g out of the box?

Because the advertising for the space on a computer is based upon the decimal system (1 GB = 1,000,000,000 B) instead of the binary system (1 GB = 1,073,741,824 B). So, the actual, real data space is about 59.6 GB.

64,000,000,000 Bytes is 64 GB, but only when you're talking in the decimal system. And because that makes it look bigger, that's what marketing uses, and it's technically & legally correct.

But your device is going to display it based upon binary calculations. So, 64 billion Bytes, when you convert it in in binary, is about 59.6 GB.

Since the iOS uses about 4 GB of space, it makes sense that your iPhone 6 Plus (64GB<dec>/59.6GB<bin>) has the correct storage space.

Reference: http://en.wikipedia.org/wiki/Gigabyte

In addition to what TJBUSMC1973 provided you, here is a support document from Apple that may help you understand it as well. How OS X and iOS report storage capacity - Apple Support