Why every Sunday rather than less frequently? The Catechism of the Catholic Church answers:
I. THE EUCHARIST - SOURCE AND SUMMIT OF ECCLESIAL LIFE
1324 The Eucharist is "the source and summit of the Christian
life."136 "The other sacraments, and indeed all ecclesiastical
ministries and works of the apostolate, are bound up with the
Eucharist and are oriented toward it. For in the blessed Eucharist is
contained the whole spiritual good of the Church, namely Christ
himself, our Pasch."137
1325 "The Eucharist is the efficacious sign and sublime cause of that
communion in the divine life and that unity of the People of God by
which the Church is kept in being. It is the culmination both of God's
action sanctifying the world in Christ and of the worship men offer to
Christ and through him to the Father in the Holy Spirit."138
1326 Finally, by the Eucharistic celebration we already unite
ourselves with the heavenly liturgy and anticipate eternal life, when
God will be all in all.139
1327 In brief, the Eucharist is the sum and summary of our faith: "Our
way of thinking is attuned to the Eucharist, and the Eucharist in turn
confirms our way of thinking."140
Yes, Mass is offered daily in most parishes, sometimes multiple times per day and in multiple languages.
Yes, Catholics are obliged by canon law to attend mass at least once a week (Sunday obligation), but to be honest, many don't. In most parishes the Sunday obligation can also be met by attending a vigil mass on Saturday evening or a late mass on Sunday evening as well as a choice of mass times on Sunday morning. There are also several holy days (varies by diocese) that are also obligations.