Still wondering WHY?
They told you: there’s money in it. “Acquire” a genuine $150 Lego set (small potatoes). Flip it for $3000 (if the market demands) on an auction site when it goes out-of-stock/out-of-print.
Baseball cards and comic books are extreme examples.
People still want the toys/artifacts of their youth and will pay for it! :-)
High price and limited supply vs significant demand.
There are many sets geared toward adults, with 3-digit price tags. With that cost, limit, and interest there may be many willing to pay a steep markup for a missed set. (Still kicking myself for not getting the Robbie House kit from the Architecture series. Would pay a good price for it.)