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Probably because Nvidia and AMD want to focus on producing the chipsets and let someone else do the coolers, the assembly and the retail distribution.
Generally the only things that matter much are the clock speeds and amount of memory.
Similar story for motherboards, which are mostly an assembly of a stock Intel or AMD chipset and a bunch of standard sockets. Once you know the size and features you want, all that really matters is whether it's well made.
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