People don’t fast now like they did in the time Jesus preached this sermon. Then it was a regular feature of religious practice, like prayer and giving. Today it is usually called for in extreme circumstances. We are more likely to fast so we can fit in our clothes than for prayer, but it is still done.
As in the cases of prayer and giving, fasting can be an opportunity for hypocrisy. I suppose any act we are supposed to do for God can be turned into an opportunity for hypocrisy. If you talk to people who are not part of the church about the church, it won’t take long for the subject of hypocrisy to come up, so we must be pretty good at it.
I might be asked to sing a song in church, or to be part of the praise team. I can do it as an act of worship or I can do it as an act of hypocrisy. Same with any other thing that we do “for God”.
We can take an act that should be done in humility and instead do it in pride.
Jesus has the better way to do it.