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@gray17 @wollman @b0rk I think there may be a collection of reasons on Unix's original very small machines:
* general context switch overhead between the kernel and user processes.
* user programs could get line-based input so only had to wake up infrequently, not every character for even lower overhead.
* there were no shared libraries, so basic line editing took less overall code space in the kernel than a copy in every application.
* the kernel's a central point so everyone did it the same.