It's really not - there is a ton of tax law that relies on e.g. the fair market value of hard-to-price assets or if all else fails and a penalty is due, there's an entire section of the CFR on how circumstances surrounding the underpayment can reduce or eliminate the liability.
https://www.law.cornell.edu/cfr/text/26/1.6664-4
If you've only ever filed a personal tax return, you're dramatically under-appreciating how complicated business taxes are and how much grey area there really is. Did you know you can pay your 10-year old to work for you as a means to avoid taxes? Try looking up the dollar amount where avoid turns to evade... there isn't one. The amount paid just has to be "reasonable and justifiable" and the work they perform has to be "work necessary to the business".