Because POSIX says that there shall be such a shell.
The more complicated answer: many people need to write shell scripts which will be portable across many systems. That is why POSIX specifies the shell and utility commands in great detail. Most scripts are written in Bourne shell, and because several important programming interfaces ( make(1), system(3), popen(3), and analogues in higher-level scripting languages like Perl and Tcl) are specified to use the Bourne shell to interpret commands. Because the Bourne shell is so often and widely used, it is important for it to be quick to start, be deterministic in its behavior, and have a small memory footprint.
The existing implementation is our best effort at meeting as many of these requirements simultaneously as we can. In order to keep /bin/sh small, we have not provided many of the convenience features that other shells have. That is why the Ports Collection includes more featureful shells like bash, scsh, tcsh, and zsh. (You can compare for yourself the memory utilization of all these shells by looking at the ``VSZ'' and ``RSS'' columns in a ps -u listing.)