Why should they? As in, materially, how does society benefit from that? How does the democratic decision-making tool become more useful from it? I consider democracy to be a decision-making process, so I don't care for vague idealistic assertions like "every adult should have the right to vote" unless there's a benefit from it. And allowing an explicitly anti-liberal, anti-democratic, bad-faith opportunist (and fascism is explicitly and openly all of those) to vote is harmful to the democratic process and increases the odds of it making a bad decision.