Designers need to be paid for their work, prototypes are necessary and expensive, manufactures need to be verified, vendors have to be confident in taking risk on the board, PCBs are expensive due to chip shortage (which is supposed to be improving), QC is difficult for small teams, the trend of brass weights makes production and shipping very costly.

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This isn't even everything that makes the production of a custom board expensive, add all these up and you can see why it's an expensive hobby. Keep in mind many of these boards are running on thin margins too.

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Your "simple aluminum" observation is like saying "the mona lisa is just paint on a canvas why does everyone care about it you can get those parts anywhere"