Why is intellectual work not considered servile work?
If intellectual work would be considered servile work then prayer would be considered servile work also. Prayer is the raising of our hearts and minds to God.
I can still recall Archbishop Raymond-Marie Tchidimbo preaching one day and in all humility stating that the hardest work for him to accomplish was to pray continuously. Mgr Tchidimbo spent 8 years in prison for the faith and was a real contemplative person who never ceased to pray.
Even St. Paul tells us to pray without ceasing.
16 Rejoice always, 17 pray without ceasing, 18 give thanks in all circumstances; for this is the will of God in Christ Jesus for you. - 1 Thessalonians 5:16-18
Praying continuously requires a lot of intellectual work!
Meditation is a form of mental prayer consisting in the application of the various faculties of the soul, memory, imagination, intellect, and will, to the consideration of some mystery, principle, truth, or fact, with a view to exciting proper spiritual emotions and resolving on some act or course of action regarded as God's will and as a means of union with Him. In some degree or other it has always been practised by God-fearing souls. There is abundant evidence of this in the Old Testament, as, for instance, in Psalm 38:4; 62:7; 76:13; 118 throughout; Sirach 14:22; Isaiah 26:9; 57:1; Jeremiah 12:11. In the New Testament Christ gave frequent examples of it, and St. Paul often refers to it, as in Ephesians 6:18; Colossians 4:2; 1 Timothy 4:15; 1 Corinthians 14:15. It has always been practised in the Church. Among others who have recommended it to the faithful as Chrysostom in his two books on prayer, as also in his "Hom. xxx in Gen." and "Hom. vi. in Isaiam"; Cassian in Conference 9; St. Jerome in Epistle 22; St. Basil in his "Homily on St. Julitta, M.", and "In regular breviori", 301; St. Cyprian, "In expositione orationis dominicalis"; St. Ambrose, De sacramentis VI.3; St. Augustine, Epistle 130, nos. 5-7; Boetius, "De spiritu et anima", xxxii; St. Leo, Sermon 46; St. Bernard, "De consecratione'", I, vii; St. Thomas, II-II.83.2.
The writings of the Fathers themselves and of the great theologians are in large measure the fruit of devout meditation as well as of study of the mysteries of religion. There is, however, no trace of methodical meditation before the fifteenth century. Prior to that time, even in monasteries, no regulation seems to have existed for the choir or arrangement of subject, the order, method, and time of the consideration. From the beginning, before the middle of the twelfth century, the Carthusians had times set apart for mental prayer, as appears from Guigo's "Consuetudinary", but no further regulation. About the beginning of the sixteenth century one of the Brothers of the Common Life, Jean Mombaer of Brussels, issued a series of subjects or points for meditation. The monastic rules generally prescribed times for common prayer, usually the recitation of the Office, leaving it to the individual to ponder as he might on one or other of the texts. Early in the sixteenth century the Dominican chapter of Milan prescribed mental prayer for half an hour morning and evening. Among the Franciscans there is record of methodical mental prayer about the middle of that century. Among the Carmelites there was no regulation for it until Saint Theresa introduced it for two hours daily. Although Saint Ignatius reduced meditation to such a definite method in his spiritual exercises, it was not made part of his rule until thirty years after the formation of the Society. His method and that of St. Sulpice have helped to spread the habit of meditating beyond the cloister among the faithful everywhere.