> wouldn't it make more sense to focus on the big fish first?
Of course, but the reason they don't is that the tax system is a psychological control mechanism disguised as "doing good" (I make that assertion by looking at the utter amount of careless, unaccountable waste in government despite the endless gaslighting about "muh roads").
Why? Because if you can keep the majority in the psychological bracket of being the property of their government (purely via suggestion), you guarantee some percentage of cash flows. The "big fish" are smart and financed well-enough to circumvent (legally, in many cases) the system, while the little fish are ripe for catch and require far less worms/fishing poles to track down.
This is why a flat tax rate with a floor is necessary. If you don't make above a certain amount per year (e.g., at or just above the poverty line), you don't pay any taxes. Everyone above that pays 10%. The government, as they should, adjusts their budget accordingly instead of treating citizens like an ATM. What the money gets spent on is transparent, itemized, and readily available for you, me, and everyone else. This would solve all of the unnecessary red tape and procedure, fearmongering, etc. And that's precisely why it will never happen.