Unlike nginx or Caddy^H^H^H^H^H, the Apache design and community encouraged writing modules (roughly akin to "middleware" in the modern server-side web stack) that hooked directly into the web server, rather than just using it as a static host + L7 proxy.
Furthermore, these modules could themselves expose language-specific APIs; ergo `mod_perl` and its ilk, which provide a bunch of useful building blocks for a full-stack webapp but are entirely specific to Apache's module API (vs. something more standard like FCGI, WSGI, etc.)
I've worked at some shops with heavy investments in Apache-based application servers and the cost of moving away from HTTPD modules and towards a more standalone, Apache-free implementation was generally quite high. Usually you need some other forcing function (rewrite in a new language, microservice decomposition, etc.) to justify the investment.