A long time ago a guy named Richard Stallman decided all software ought to be open source and free. So he started writing a bunch of UNIX utility programs, some new, some clones of old ones.
So you may ask, why write another one? Well, why have more than one of anything?
The reason there are some many similar commands between Linux and UNIX is the same reason there are so many similar commands between Linux and Windows. Think about it, there's rm/delete, cd/cd, ls/dir, etc. There's still such commonality between these systems (which work in starkly contrasting ways) that a number of commands to maintain them are almost identical. Since Linux is meant to be very similar to UNIX, it's even more so in that case, but this applies generally to all operating systems.
There really isn't a huge deal of uniqueness between operating systems, they're almost fundametally all the same.