The set is never quite complete, and finishing it costs more.
Sets are built so completing them needs more purchases, trades or luck than one copy provides.
The missing pieces: a second copy, a trade, more pulls or more packs. You are paying to close a gap the game left open on purpose.
In Pokémon Red and Green (1996), some creatures appeared in only one version, so completing the set meant trading with a friend who owned the other cartridge over the Link Cable. Players still paid a one-off price, and nobody felt cheated, even though the design sold extra copies. Gacha games turn the same urge into a lottery: collection sets are engineered never to quite complete, and the last items can be exponentially harder to pull. Japan banned that version (complete gacha) in 2012.
When completion is possible within the game you bought and trading with friends is an extra, not the only route.
Pokémon Red and Green (Japan, 27 February 1996) split monsters across two cartridges; producer Tsunekazu Ishihara built version-exclusive creatures and the Game Boy Link Cable specifically to force players to trade to complete the set. Gacha (from 2012) industrialised the idea. Japan's Consumer Affairs Agency banned kompu gacha in May 2012.
In May 2012 Japan's Consumer Affairs Agency banned 'kompu gacha' (complete gacha), where the last item in a set became exponentially harder to pull.
Historical prices and revenues are in US dollars at the time unless marked A$.
Pokémon Red and Green (Japan, 27 February 1996) split monsters across two cartridges; the Link Cable was built to force players to trade to complete the set.
Japan's Consumer Affairs Agency ruled 'kompu gacha' illegal in May 2012.
Covered in Video Games & Money: Ep 10 · Handheld, Ep 30 · Gacha.