With just 10 pieces of 3090s you are already at 1GH on the Ethash algorithm, why aren't people doing this instead of having so many GPUs around, taking so much space and having to use more risers and more motherboard, why are people not going this way?.
One reason is they are starting with enough money to build a rig smaller than what you recommend. I too started from a single GPU back then until it became 5 of them. Another reason is the supply, there are scalpers around in every corner of the earth, new stocks are always sold out or if available it is higher than MSRP. There are lots of reasons that you cannot do what wanna do, unless you buy it from scalpers in order to build your dream rig, or buy it used and feel it's remorse later on.