It is an open source project, I am guessing that one or more contributors decided they wanted the built in be web server.
There is nothing stopping others getting involved if there were other things they want to see in future PHP versions.
There is nothing stopping others getting involved if there were other things they want to see in future PHP versions.
Furthermore this kind of web server would only be useful for the most basic of projects. As soon as a person dips their toes into anything remotely complex, they're going to need their development server to mirror the production server as closely as possible, especially with things like php.ini and rewrites. So why not just set it up like that to begin with?