Getting a cluster to guarantee that something happens excatly once is very hard, but doing things exactly once is an extremely useful property in banking. Using a single earthquake-proof computer with redundant everything, including hot-swappable RAM and CPU gives you similar reliability while making it much easier to achieve many desirable properties. Even if you can do the same in a cluster, spending more on hardware to reduce software complexity is worth it when bugs might cost you millions or billions of dollars.