That's because the hiring manager doesn't need to understand tax rates to do his job. He plugs the numbers into the black-box HR system, and the system spits out dollars, that he pays employees with.
The reason for why you have no idea how much the government will take in taxes, is because income taxes are progressive, and are charged based on total income earned per year.
So, someone working at a $200,000/year job for a year, will have a tax rate of ~26.5% (I'm ignoring Social Security).
But if they were unemployed for 9 months, and started working that job on October 1st, they will have a tax rate of ~16.5%.
What this means is that when you start a job, you have to guess what your likely annual tax rate is going to be, and set up withholding for that amount.
If you underpay by a large enough sum, the IRS will send you a bill, with an extra penalty next April. If you overpay, the IRS will send you a tax return.
Expecting the hiring manager to know the tax stuff would be like expecting the dev team to know how much income tax the business is going to pay.
This should be a bipartisan issue, but neither party seems interested in tackling it. A cynic might argue that this is because both parties are interested in maintaining the status quo, their leadership being full of people who benefit from it (or friends of such people). It also helps ensure that running the country is expensive and inefficient, making progressive policy harder to implement.