A) If economics were the only reason for doing anything then getting a job stacking shelves in a super-market would make more sense than producing rpg books.
B) Everyone in a group buying a copy of the core book may be expected of higher crunch games where you are constantly referring to the rules but in 35 years in the hobby I have never once played in a single session where more than two books were present at the table and even that has been vanishingly rare.