I'm not saying they don't; Especially on the upside, tax rates matter a lot, and the potential upside effects how much investment a startup gets, at all stages.
I'm just saying, when you are still trying to scramble up to profitability, complex tax laws are a big deal. Certainly under $10K/year profit (and probably for a while further) you are going to be spending more on tax related accounting and planning than you will pay in taxes. You can't just ignore it because you aren't making any money.
It's not just paying the accountant at the end of the year; Especially in lower-margin businesses, how something is taxed can make the difference between profit and loss. You've got to run all your ideas by the tax expert. This is expensive, as both I and the tax expert have deep domain specific experience that needs to be at least partially shared to figure out if a particular idea can work or not.
Yeah, once you've got two million in profit a year, the cost of that tax person is probably a good bit less than what you are paying in taxes, and I imagine you care a lot more about the rate. I'm talking about those of us who are still in the red or only a little in the black.