If you're doing one of those simple problems (e.g. cancer test has X false positive rate and Y false negative and the prior rate of cancer is Z) then you're not getting confidence intervals because you're assuming that X, Y, and Z are known 100% correctly. If you have confidence intervals to input for X, Y, and Z, you will output confidence intervals as well.

Similarly, 2+2 will always give you 4, without a confidence interval attached. But if you add two numbers with confidence intervals, you'll get a number with a confidence interval, probably after you make some assumptions about independence or about what your intervals mean.