Because it's binary. Binary is good.
Q: "Why climb Mount Everest?" A: "Because it is there."
Sometimes also in form of "Because I can", which is almost equivalent
Isn't it the other way around? The illiterate masses think that 1 GB is 0.9313 Gigabytes, while 1 GB really is 10^9 bytes and 1 GiB is 2^30 bytes, so the conclusion is that 1 GiB is the capacity you (and lots of software) are mistakenly refering to as 1 GB.
This is the way I see it. Besides, I like the fact that in the normal scenario where I'm assessing wether some chunk of data will fit the free space I can easily convert the units in my head. Will a file the size with reported size of 3221225473 fit reported free space of 3.0 gigabytes?
What next? I didn't get the amount of free space stated on the cover of the box after I formatted the disk with a file system?