Long story short, the thinking is probably similar to the situation with altar servers: Ideally, only a seminarian who has received the first 4 Minor Orders (Porter, Lector, Exorcist, Acolyte) has a right to serve Mass. But since most chapels don't have seminarians available, boys in cassock take the job.
Yes, I believe that's exactly what the thinking is. But it just seems this crosses a line. Now, most Traditional Catholics object to the Novus Ordo practice of doling out these liturgical functions to lay people, such as having Lay Lectors. So is seems that this crosses a new line. But that's an emotional reaction, and it's important to understand where said line is ... based on principles.
Here are the Catholic principles.
Liturgical Prayer, properly speaking, is the public prayer of the Church, and so in Liturgical Prayer it is the CHURCH praying and not just individuals. So, the Mass, Sacramental Rites, non-Sacramental Rites (such as which appear in the Rituale Romanum), and Divine Office are Liturgical Prayers. Not that someone might not be able to pray these privately out of devotion, but when done privately they are not Liturgical in nature.
Consequently, Liturgical Prayer requires someone in a capacity to actually represent the Church. So, for instance, women cannot exercise Liturgical functions, properly speaking, because they cannot serve in the capacity of "heads" to represent the entire Church body.
But, what's more, no mere layman can represent the Church and pray on behalf of the Church. I'll go on in a subsequent post.