[mindslight]

The argument is that all of the bookkeeping and accounting required to adequately deduct expenses creates an undue burden on small businesses, especially when it comes to mandatory accounting paradigms like depreciation. This is effectively demonstrated by the conventional wisdom of "just hire an accountant", which will run you at least $500, in addition to needing to get up to speed on how to track expenses and what to track.

A single member LLC defaults to a "disregarded entity" for income tax purposes, meaning it has no bearing on taxation or this discussion. And opting into being taxed as an S-corp (or partnership) creates even more paperwork.

Having said that, one of the great inequalities of our tax code is that individuals are not allowed to take most deductions that business take. It's patently ridiculous that a W-2 employee driving their car to work every day cannot deduct that necessary expense to have worked. But really the whole filing process needs to be simplified, the IRS needs to release tax forms as executable programs (eg python) rather than this archaic "add the lesser of line 25 or line 27 to line 3 from form 1138", and the formulas need to be set and published ahead of the tax year as to stop being ex post-facto laws.