Some people do have complex financial situations. It's not as simple as that.

For example in the UK (and maybe US?) you get tax relief for money you put into your pensions, but there's a limit of £60k/year. Unless you earn a lot (which I do, yeay) when that limit is tapered. Except that you can also use up to 3 years of previously unused allowance. But you have to use this year's first.

Also interest is taxed, but you can put up to £20k/year into an ISA which isn't. And if you still want to avoid some tax you have kids ISA's and even pensions!

Then there are also startup investment schemes that save you some tax. Those seem to be not worth it, but you get the idea - it can be complicated. Especially if you are near one of the many tax/benefit thresholds.

The marginal tax rate in the UK bounces all over the place - it's even technically possible for it to be over 100%!