No, it's just that we're in the realm of predictions and modelling, not accounting. If you're constructing a curve to forecast 50 years of interest rates from a limited set of instruments, you're already accepting a margin of error orders of magnitude greater than the inaccuracies introduced by floating point.

The models also use transcendental functions which cannot be accurately calculated with fixed point, rationals, integers etc.

It's not like decimal or fixed point does not suffer from rounding errors either. In fact for many calculations, binary floating point gives more accurate answers.

In accounting there are specific rules that require decimal system, so one must be very careful with the floating point if it is used.

Excel uses IEEE-754 floating point, so I don't get what you mean with the spreadsheet comment. It has formatting around this which rounds and adds currency symbols, but it's floating point you're working with.

Rounding error doesn't matter on these types of financial applications. It's the less glamorous accounting work that has to bother with that.

They're not rocket science, but they're unnecessary, and would still be off anyway. Try and calculate compound interest with your fixed point numbers.