Exclusive variants and license-exclusive minis create a second demand market.
Some sets have minifigs that cannot be obtained any other way. Ninjago characters in specific armor variants. Stranger Things' Eleven. Ghostbusters' Egon Spengler. Seinfeld's Kramer. These exclusive figures are independently collectible — sometimes worth more than the set itself on the secondary market — which creates a second buyer pool: minifig collectors who don't care about the set, only the figure.
A plain set with a rare exclusive mini can outperform a flashier set with no exclusive minis. The minifig collector market is large, active, and willing to pay premiums. They sometimes buy the whole set just to harvest the figure, then resell the rest at break-even. That demand pulls the entire set's price up.