Presumably because the pastor or liturgy committee thinks that dry fonts will foster the "desert experience" and the penitential nature of Lent. Regardless of motive, the holy water fonts should not be emptied until after the beginning of the Triduum on Holy Thursday. (Lent ends and the Triduum begins when the Mass of the Lord's Supper on Holy Thursday begins.) On March 14, 2000, the Vatican's Congregation for Divine Worship and the Discipline of the Sacraments released a statement on the practice, saying the following:
Holy Water Fonts and Lent
The encouragement of the Church that the faithful avail themselves frequently ... of her sacraments and sacramentals is to be understood to apply also to the season of Lent. The "fast" and "abstinence" which the faithful embrace in this season does not extend to abstaining from the sacraments or sacramentals of the Church. The practice of the Church has been to empty the holy water fonts on the days of the Sacred Triduum in preparation of the blessing of the water at the Easter Vigil, and it corresponds to those days on which the Eucharist is not celebrated (i.e., Good Friday and Holy Saturday) [source].