There's a bug report for httpd dating back to 2011[0]. The nginx mailling list also has a grumpy person contemporary with that[1].
My guess is someone thought "httpd is a server running somewhere without a monitor attached, why on earth would it get a SIGWINCH!? surely it's available to use for something completely different", not considering users running it in the foreground during development. Nginx probably followed suit for convention, but that's pure speculation on my part.
Also that was before docker really took off (I'm not sure if it was around in 2011 yet; still in it's infancy maybe). Running it in the foreground didn't happen as much yet. People were still using wamp or installing it via apt and restarting via sudo.
[0] https://bz.apache.org/bugzilla/show_bug.cgi?id=50669
[1] https://mailman.nginx.org/pipermail/nginx/2011-August/028640...