Because they used to have Apache in base from 1998 to 2014 (the 5.6 release) and found it frustrating to maintain? Given the developers and what they do with the operating system, I can understand why they would consider having an HTTP server in base something within the scope of the project.

It is an open source project with both commits and discussion happening in the open (tech@ in particular), so the barrier to understanding is reasonably acceptable. At some point it was also debated to move to nginx, but in the end httpd(8) was championed as a natural offshoot from relayd(8) which was already in base.