Earlier implementations of the Unix kill command did not allow names, only numbers, so many people (deeply familiar with Unix) know the numbers as well as or better than the names. Plus, it's shorter.
WRT shorter: Magic numbers don't just suck in programming.
7th Edition AT&T Unix did not allow signal names (http://plan9.bell-labs.com/7thEdMan/v7vol1.pdf, search for "extreme prejudice" -- I still remember many of the little gags in the early manpages).
That was a mainstream release in the mid-1980s. Even the basic utilities like kill(1) were incompatible back then, so if you worked on both BSD and AT&T systems, it was easier to use the compatible subset.
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Regarding magic numbers: in general, yes, to be avoided. But my usual use of kill -9 is in exasperation, from the command line, and clarity for others is not a priority. I admit, in a script, kill -HUP is to be preferred to kill -1. But even in a script, I'd say kill -9. This usage thing seems to be complex.