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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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I think the major difference between the different brands putting out the same card is the cooling performance. Also they offer different types of warranty.
I just built a new pc and it runs very cool (altho ambient room temp has been fairly cold recently) and fairly quiet. The only thing making noise is one of the case fans (I just reused my old knackered case). I also did as someone said above - bought a pretty high end processor, haven't overclocked it, and went for a slightly overkill heatsink on it (not sure if this cooler would fit in mini itx tho). Doesn't make any noise at all and stays very cool.
Aorus pro WiFi board (onboard WiFi adapter seems extremely good, ethernet havent tested much)
i9 9900k
Dark rock pro 4 cooler
Gigabyte 3070 "gaming oc"
2x8gb of ram running at 3200 (can be clocked faster on this mobo)
An EVGA 750w PSU which is quiet enough for me altho I have got a better PSU lying around which I might put in for even less noise.
1tb Samsung SSD is all the storage for now.Whole thing cost around £1300 altho I got extremely lucky finding a GPU. If you can fit this into a smaller case I'd highly recommend
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