You're saying the exact same thing. Bitcoin is created by special coinbase transaction with no predecessors, each block contains at most 1 of those, if block pr

You're saying the exact same thing. Bitcoin is created by special coinbase transaction with no predecessors, each block contains at most 1 of those, if block proposal was easy then peers would be incentivized to create money out of nothing. Double spends are just another, smarter, subtler way to "create" money, by exploiting inconsistent versions of the same supposedly-shared state. It all boils down to scarcity in the end, that's a necessary condition for a currency. Value is scarce, so anything representing and storing it must be so too.