Because technically, it's a scanner + printer and not a mover. It's just that the original is deliberately destroyed every time (unless something goes wrong *looks at Miles*).
IIRC, it's even mentioned that people going through the transporter are cached. Like, they store snapshots, so you can compare the changes between older you and newer you. You can also store a person in buffer digitally, meaning you can copy and print as many as you want.
Anything But Tangerines