Maybe the company was betting at the time on an integrations marketplace, or a lot of partnerships, so a robust integration platform was built. Then the company moved on to another bet after only writing one or two integrations on the new platform. Nobody over-engineered here. Everything was built to spec. But three years down the line the new dev is going to get assigned a bug in one of the integrations and lament how the code was over-engineered. Hindsight is 20/20.
Lots of stories from the trenches including many in this thread make this mistake. The same goes for 'tech debt'. If you weren't there, you don't know.