It's done for speed. Straightforward text replacement is so much faster than checking to see if a path is properly terminated by a slash. And remember that Nginx became popular due to benchmarks that showed that it was more "web scale" than Apache2.
In any case, it seems that nginx does try to search for .. but has a bug in the corner case where the “location” doesn’t end with a slash. I assume there’s some kind of URL normalization pass that happens before the routing pass, and if the route matches part of a path component, nothing catches the ..
If I’m right, this is just an IMO rather embarrassing bug and should he fixed.
More likely it is an omission, which could be rectified with a warning or failure running nginx -t (verify configuration).
The actual performance comes from an architectural choice between event vs process based servers, as detailed in the C10k problem article. [1]
More like because it was much faster out of the box, and came with many batteries included while Apache2 required mods to be separately install.