1.) Because if you are of the population who likes their holidays to have actual religious or historical significance, it might concern you that it's ridiculously impossible to get consensus on the true origin of this holiday.
Between accounts that claim it's a liturgical holiday celebrating the martyrdom of Christian saints, and much earlier pagan history of licentious behaviors occurring on February 13-15, and medieval claims that St. Valentine did not even have any romantic connotations until Chaucer's poetry about "Valentines" in the 14th century... and too numerous a list of other historical claims, legends and folklore from around the world all laying claims to originating this holiday, the research will leave you exhausted and wishing it were Easter already.
Yes, Valentine's Day is a hybridized holiday if ever there was one.
Which basically means -- it was created to encourage shopping (see #2, below):