At $main_work, the reason is that there's a bunch of RewriteRules which last I checked simply couldn't be done by NGINX.

OTOH, Apache suffered from the "slow loris" attack, so the whole shebang ended up being nginx sitting in front of a few front-end apache instance kinds, which sit in front of a dozen or so backend apache instance kinds.

I find it interesting that although on those servers there are 12x more Apaches than NGINX, it might get counted as a server "using nginx"...

... and that's just because the whole she-bang sits under cloudflare, which reports Server: nginx-cloudflare ;)