I worked at GE. A lot of thier software dealing with the transportation industry back then wasn’t technically complicated. There were just a lot of business rules and regulations they had to check for.
It's not at all about doing hard things. It's simply the number of things to be done and how they interact with each other. You are absolutely right that these are technically not hard problems, but they're still hard design problems that keep changing very frequently
My vertical was railroad car repairs
https://public.railinc.com/sites/default/files/documents/CRB...
Most developers aren’t doing “hard things”. Not even at BigTech