Why do you say stuck? What do you think is better at handling currency? I can think of integers (multiples of the smaller denomination) or fractions (which are quite slow), definitely not floats.
AFAIK this is only because modern CPU's have floating point units (https://en.wikipedia.org/wiki/Floating-point_unit), from memory these became common on home PCs around the time of the 486. A lot of early 3D games used fixed point because it was much faster for those that didn't have a co-processor installed.
Whether any of this is applicable to mainframes I don't know.
The main reason people use floating-point math is convenience. It's like dynamic typing: write your real quadratic equation solver with floating-point math and you can use it for everything, whether the values are 6.02e23 or 555e-9, at the expense of having to do extra computation at run-time, just like dynamic typing lets you implement a single hash table type that works for any type of value. But in fixed-point, unless you're using a pretty fancy language, you need to decide on the magnitude range of your values up front. Maybe 3e-6 to 3e5? Use 16.16. Maybe 4e-3 to 1e11? Use 24.8. It's a pain in the bohonkus. But it's faster, more portable (if reproducible results are necessary) and easier to reason about than floating point.