>Nothing precludes you from doing integer arithmetic in a dynamic language.

You would be surprised. The second you use pandas with a custom data type (let alone any other library you'd want to use) it can randomly auto convert it to a float. Furthermore identifying when it randomly converts the type on you is a pain.

>so floating point basically must be used.

Quants tend to use fixed precision types. It is like a float in every way, except base 10 instead of base 2 so there is no floating point error.