Taxes are complicated and hard to predict whether you account them before hand or later. If you're only looking at your tax liability at the start of year, you aren't really doing a complete long term budget. Many can get away with this if taxes are pretty even no matter what you do, but if they aren't, you may run into surprises. I feel like I'm at least making an attempt to estimate future tax liabilities my way, rather than just hoping for the best.
Just for example, at least some people here must be living primarily off money in taxable accounts, getting mostly dividends and maybe selling some investments, with small capital gains. They may be able to keep all of this at 0% tax due to being in the 15% bracket. But once they start getting a pension, SS, and tapping IRAs as the taxable account runs down, this is all regular income taxed mostly at 15%. Suddenly the tax part of their budget is likely to go way up.