> Because you use C or one of its derivatives instead of Lisp or one of its derivatives. In Common Lisp you can not only debug a function without restarting your program, you can redefine classes without restarting your program. It is truly awesome. You should try it some time.
There is no technical reason why it shouldn't be possible in C, if you are willing to do without many optimizations. One approach is to make a function stub that simply jumps into the currently loaded implementation. A more efficient but more convoluted way is to patch the address of the currently loaded implementation at all call sites.
The problem is that in general you can't simply replace a function without either restarting the process or risking to crash it. In general functions have some implementation-dependent context that is accumulated in the running process, and a different implementation does the accumulation in a different way. I'm not a lisper, but there is no way this is different in LISP. (And it is not because in C you often use global data. "Global" vs "local" is only a syntactic distinction anyway).
If you're willing to risk crashing your process that's okay. It's often a fine choice. And you can do it in C. The easiest way to implement approach #1 is to load DLLs / shared objects.