I would recommend The Mass: A Study of the Roman Liturgy by Father Adrian Fortescue, published in 1912, for everyone's edification.
The first half of the Mass, the Mass of the Catechumens, indeed has an instructional character, and it is noted that catechumens were admitted to it but then expelled before the Mass of the Faithful, the sacrificial part of the Mass.
At one point it was the norm for an exhortation (a homily) to be given on the scripture readings. This practice fell away, in part I've read because that for a time the scholastic formation of the clergy was not good, which also resulted in the faithful becoming weak in their faith. The Council of Trent addressed this issue by issuing decrees on the formation of the clergy and also by directing that instruction be given during Mass (the sermon), and the Catechism of the Council of Trent was issued to assist the clergy with this task.
When the Roman Missal of 1570 was promulgated by Pope St. Pius V, while perhaps most people may not have been literate, those who were educated could read and write Latin, and in many cases also speak and understand it. I've not time to scurry around for citations, but up into the early 20th century educated people were typically literate in Latin.
There is some counter reformation "ecclesiastical politics" involved with why certain parts of the Mass were not allowed to be said in a vernacular language, the situation is what it is. The fact that typically the priest on Sundays and Holy Days re-reads the Epistle and Gospel in the vernacular, followed by a sermon, and that Pope Pius XII encouraged the use of vernacular hand missals by the faithful, I believe reinforces the Church's understanding that the Mass of the Catechumens has a definitive instructional character. Since God is the author of all scriptures He really does not need them read back to him.