Big-number libraries break down if you want to do anything other than be a 4-function calculator (and honestly, even division can be questionable, since some big-number libraries use fixed base-10 and thus even ⅓ isn't accurately representable). Want to throw in a call to sin or exp? Your only realistic option is some sort of floating-point, and that quickly boils down to "do we use IEEE 754 single or double precision? Or are we in the rare case when we need quad [1]?"

[1] I believe the biggest use of quad precision is evaluating the accuracy of double precision arithmetic.