why doesnt my ipod touch 8th generation 8 g only allow me to use up to 6.4 gigs?

because of the way memory is reported on most all computers devices adn the fact that the operating system uses up someof that space.

TS2419: How OS X andiOS report storage capacity - Apple Support ...

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

Because after formatting, installing iOS and apps, that's what is left of the gross 7.8 GBs of space. If you read the disclaimers 8 GBs means 8,000,000,000 bytes, not 8,740,929,536 bytes (this is "real" 8 GBs.)

See How OS X and iOS report storage capacity.