The Subject of the Homily

From whence this almost universal notion that the only licit topic for the homily at a Catholic Mass is one of the "three", (or on weekdays, two,) scripture readings?
Putting aside the fact that far more scripture is contained in the Mass than just the lectionary readings, (and putting aside the fact that the lectionary psalm is scripture, although many priests seem to forget that...) and that the proper antiphons are even more neglected than the psalm or graduale, what is actually required is that an "exposition of some aspect of the readings from Sacred Scripture or of another text from the Ordinary or from the Proper of the Mass of the day" be preached on.

One of the finest, (perhaps THE finest,) homily I ever heard preached was on the dialogue "pax vobiscum.... et cum spiritu tuo."

But no!
I am forgetting the only sermon I remember from my childhood, (I don't recall it being tied into any text from the Mass whatsoever.)
A man had died on a Saturday evening, a professional man, the father of a schoolmate. His family had been turned away from a funeral home that evening, suggesting another funeral parlor might be more suitable, because they were black, and it would have hurt, if not destroyed their business.
I did not know such things happened.

One of our... curates? associate pastors? not sure what they were called then.... rattled the rafters, like some fiery renaissance Italian Dominican, excoriated, not just the funeral parlor itself, though they came in for a scalding denunciation, but not just them because they were right - to have done the right thing would have done their business serious damage. He thundered, it was not their fault alone but ours. Our town. Our country. Our society.

It was thrilling and frightening.
I will never forget Fr L.  - and that day, his visage like a hawk, eyes like coals, his voice ringing out like St Michael the day of Judgement, weighing us and finding us wanting, and the awful, awful truth of what he said, will stay with me forever.