The docs show this example:
legend( ('label1', 'label2', 'label3'), loc="upper left")
Presumably, you could write loc=2, but why would you? It’s much more readable to use the English word.
As to why they didn’t enumerate the values to align with the numeric keypad, I presume they weren’t thinking about the numeric keypad at the time.
Edit: It’s worth including here the full text of Joe Kington’s comment:
Actually, they were deliberately mimicking matlab’s behavior at the
time. See the “obsolete location values” section in the documentation
for MATLAB’slegend: mathworks.com/help/techdoc/ref/legend.html