What systems don’t have grep? Off the top of my head, anything vaguely POSIX compliant would have it, and anything descended from 4th Edition Unix would also have it, which seems like it would cover everything.
Toolchains for compiling systems from source is another answer, e.g., NetBSD's toolchain has sed, but not grep.
A third answer is install media. For example, NetBSD install kernels have ramdisks with sed but not grep.
A fourth answer is personal, customised systems. I create small systems that run from RAM. I run these on small computers with limited resources. When one of these computers first boots up, it may not have a "full" set of userland programs. It may not have grep. I am not inclined to use the limited space available to include grep at such an early stage if I can get by with sed.
Maybe someone deleted grep by accident? Or maybe you accidentally traveled in time to the 70's and need to use a nearby PDP-11 to calculate a way home? Lots of possibilities.
