This is the reason we have mods. Additionally, if the expansion packs required to be paid for, people would just buy them and then turn them into mods for other people to use for free.
We must also keep in mind that Minecraft is an indie game. Just because other games have expansion packs, doesn't mean it should.
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Minecraft is a periodically updated game by Mojang. As such they are their own small expansion packs. For anything that is not or will not be included in vanilla Minecraft there is the option of using mods to increase or change the experience.