In these stories, Jesus challenges the religious culture and practices of the Pharisees, not the Old Testament. His understanding is so fundamentally incompatible with the Pharisees that if he tried to reform their their views, he would only destroy them.
fasting
31 Matthew 6:1-6; 6:16-18 Giving, Praying & Fasting
Being religious is no guarantee that you are genuinely following God. Whatever you define as obedience to God (being in full-time ministry, church attendance, praying, fasting, giving to the poor, adopting social justice causes), Jesus says: stop and ask yourself who are you doing it for?