Why could no one tell me this?
I could have told you that. There are 8 bits in a byte. So you'd have to divide the number you thought you should be getting by 8 to see the real number. Another thing that may be confusing your calculations is that your modem doesn't really connect at 56kb, it's really only like 53, which is, IIRC, an FCC limitation on transfer speed over phone lines at the frequency that voice data (standard phone calls, analog modems) uses.
It's the same reason I laugh when people brag about having Gigabit (not byte) ethernet cards in their PCs, but don't realize that the bottleneck is still their DSL connection.
Or when people wonder why their 200GB drive only shows up as 187GB in Windows. They don't realize that the hard drive manufacturers are using base 10 to derive the size of their drives (1000 bytes = 1 MB, 1000 MB = 1 GB), when the actual computer is using base 2 (1024 bytes = 1 MB, 1024 MB = 1 GB).
I think what happens is that the people who weren't around when this technology came to be never learn the why of it. It's the same thing from people that complain about Windows XP being hard to use. I just laugh and tell them to try getting a game to run under DOS, with sound and a game controller, and then get back to me about how "difficult" XP is.