Why most financial calculator apps suck?

Because the input variables are so different depending on your personal circumstances. Depending on how clever you accountant/tax professional is, you could be paying a very different amount in taxes/interest.

I might have misunderstood your question, as you didn't say which apps and how they suck. Then you start talking about UI and I get lost.

I have tried to build something simplier at Goodly

www.goodlyapp.com/calculator

What visualizations are not available?