To make money for all the businesses selling flowers, chocolates etc etc etc.
Mothering Sunday was a Christian festival in Lent followed by both Catholic and Protestant churches in the 16th century. It was a day to return to the mother church, the family church you grew up with and for families to get together. It was taken to the USA in the 1920s and turned into a secular festival called Mothers Day and designed to make money for florists, confectioners, card makers etc.
Fathers Day started as a feast celebrating fathers on St Josephs Day and was brought to the Americas by the Spanish and Portuguese. Again, around the time that Mothers Day became a secular commercial exercise in the USA, Fathers Day became the same.
Another one is St Valentine's Day. This started as a religious festival centuries ago but commercial practice in the USA has seen the name change from Saint Valentine's Day to just Valentine's Day and all the traditions associated with it are in the dustbin.
Halloween, a centuries old religious festival has gone the same way and Christmas and Easter are doing the same. There are still religious undertones to Christmas and Easter but not to Mothers Day, Fathers Day, St Valentines Day or Halloween.