I don't think this is an ethical issue, I think it is a psychological one, and at root it is strategic rather than moral.
Democratic societies, more properly Liberal Democracies (as other kinds of cultures also vote), are generally Capitalist in orientation. Capitalism hopes to encourage excess production by leveraging basic human psychology.
Many people genuinely cannot be adequately motivated to contribute at the levels we expect them to out of concern entirely for themselves, or for some more abstract goal. But there is still a basic human drive to provide for and take care of others to whom one has a strong emotional connection. If most of our property did not go to those people either in life or upon death, I think the Capitalist threads in our cultures would have problems focusing workers who are basically pro-social, but not extremely broad in vision.
Since Liberalism is often in origin Christian, mutual support and giving is the dominant family morality, as contrasted with filial piety, religious obligation, patriarchal loyalty, or name honor. This compulsion to have someone to support, or with whom to pool resources in mutual support, runs so deep that our traditions have mechanisms for replacing families as objects for this attachment when children do not fit into them well. For instance, religious orders or communes construct 'families by intention', which often usurp those sentiments and inherit in place of the next of kin.
So without inheritance of some sort, individuals would simply arranged to give away their property before death, which would be a messier version of exactly the same system, since they might miscalculate and leave themselves destitute in old age. In less options-oriented societies, the elderly traditionally do this on purpose, becoming dependent upon their children in old age on purpose.
Since Liberal Democracies also believe in relative freedom of movement, if we don't make some sane solution easy, the successful would take their property with them into a similar society more conducive to their notions of propriety.