I know it is stated in the Canon law. Also, stipends don’t go to the priest sometimes; some dioceses have rule that stipends from baptism, marriage, etc goes to the parish.
But stipends give rise to the impression that one has to pay to get the sacraments. Stipends are said to be just stipend, voluntary, not fixed fee. (Although Pope Francis reminds us several times that he knows there are parishes that demand a certain fee in writing, for which he forcefully condemns as a scandal). But giving someone money after we ask for a certain service looks just like paying for service.
Stipends need to be removed from our church laws. It has been there for 2000 years, but Ecclesia semper reformanda. The church always has to change.