Flat taxes do not work, because the established system is one where there are two classes globally - the consumer class, that is productive to consume resources with the fruits of their labor, and the capital class, that owns the means of production and use the wealth that creates them to make more wealth.

If you take 30% of everyones income, the working class loses standard of living and upward mobility while the capitalist class reevaluates their investments to insure that, despite a 30% annual expense, you are growing your wealth regardless.

It is also a dangerous precedent. If your intention to tax sufficiently high to start peeling back the democracy distorting wealth inequality, the wealthy will just flee you, and you must remember - as they exist today, they are the arbiters of society, and dictate who lives and dies through economics. If anyone outright attacked them like that, you should expect a war in return.