For the reason most individuals do not have or take the time to understand the topic one will vote for, or simply the topic is too complex to understand all implications, in a reasonable time, the general vting system does not work, at least not for large group voting, eg government elections and so on.
This is why Bitcoin uses Proof-of-Work, and Steem, Delegated Proof-of-Stake. Not saying they're perfect, but they seem to work fairly well in massively open, permissionless environments where "Proof-of-Identity" is likely impossible.
You say they work fairly well. I doubt that to be honest. We try to believe they work fairly well, but I actually think the government and referenda voting works better than the voting system you refer to wrt blockchains. But that is my humble opinion :)