It's not inherent (for example private blockchains exist and are being used, but private blockchains may as well be a centralized system anyway), but it is a soft requirement. You may create a Blockchain without mining rewards, but then there is no incentive to do work and you are much more vulnerable to 51% attacks. You could argue that federated servers already perform work out of the goodness of their hearts, but in the case of federated networks the work being done is much less than PoW blockchains, because only the first node to validate a block does useful work, all the other mining done is worthless. That in turn means that you are hammerring your server (if you aren't, see above comment about 51% attacks) for effectively 0 reward. If you add the fact that blockchains as they exist must always be dealing in some sort of "value" exchange, and it means blockchains always are just money exchange schemes. Their volatility also turns them from stores of money to speculation a instruments and then it's even harder to build community on it (I suggest the second part of the Line Goes Up documentary on yt for the community aspect of blockchains).