Huh, I'm fairly sure the IRS isn't just blindly trusting what people report as the cost basis, but maybe that only comes up during audits.
But you understand that a lot of the documented reasons the tax code is bad, is lobbying by the tax prep industry, right? Their business model is built on it being more dangerous to do taxes yourself, than to pay them $50+, so they lobby against simplifications to the tax system, and similarly lobby to support laws that increase penalties, and ensure that new reductions aren't automatic.
The IRS does trust your cost basis, but I have no doubt there is some backend check that flags things that are wildly off of what is expected.
But I don't think the tax prep industry lobbied for how RSUs cost-basis is captured. That's just a byproduct of the US tax code and requirements for reporting on 1099s from brokerages.
But you understand that a lot of the documented reasons the tax code is bad, is lobbying by the tax prep industry, right? Their business model is built on it being more dangerous to do taxes yourself, than to pay them $50+, so they lobby against simplifications to the tax system, and similarly lobby to support laws that increase penalties, and ensure that new reductions aren't automatic.