Because every programmer should at least once. Writing a compiler/interpreter is the ultimate exercise for any CS student/professional. that and maybe a file system/database or toy OS or server, because it encompass most aspects of CS.

Anybody that can do this things can probably take on any programming job, because they require a fundamental understanding of how computers and programs work.

Furthermore, It also help when suggesting realistic features to other language developers, or write proposals, or assess languages themselves.