Some cultures do have such arbitrary numbers codified in their religions, as it happens - 10% for Muslims, 20% for some Christians, etc.

Secularly speaking, tax law in most countries is laid down with certain public thresholds, defined through open process by democratically-elected representatives, and everyone is supposed to abide by them; but the rich can afford armies of professionals poking and prodding these laws until they find loopholes, contradictions and secret hatches. Obviously the legislator should carry some of the burden for not explicitly forbidding these escamotages; but it's similarly obvious that this sort of activity has reached levels that, even where ostensibly legal, are patently immoral and against the spirit of the law, and that's not the legislator's fault.

In a lot of cases, these activities are not even legal, in fact; they are just not prosecuted because authorities lack manpower for enforcement. This is why public leaks often results in new charges being brought forward: because someone else did the job and the taxman can piggyback on that. Responsibility for illegal activities lies squarely with the people engaging in them.