The underlying issue isn't really laziness or being too busy or any of that. The real reason so much material is in demand is that "the game" isn't primarily about what is going on in the campaign anymore, its about having new rules widgets with which to do cool stuff. The PHB has been out long enough that most of the Lego set parts are very familiar now and there are only so many things you can build with the same bucket of bricks.
This is because gaming culture at large has moved away from what the characters are actually doing as the primary area of interest. The focus has moved largely on what can be accomplished via synergies in the published rules. In order for that game to remain interesting, constant expansion sets are needed to provide different bricks that can assembled in new combinations.
If the game revolves around what is happening in the fictional game space then very little published material is actually required. The B/X boxed sets will keep you going for many years because all the really interesting bits are imagination based.
Once you accept that for many players, the primary area of interest will be getting an AC of X by Y level, or how to legally pump out ZZZ damage per round, then you will see how important regular officially released products are and how new generated home content just won't do the trick. After all, you can't go online and crow about beating the system and how your character is by far the baddest in the land if you have to use home brewed material to get it.
This is also an excellent point and another axis/variable to throw into the equation.