I suspect a lot of the development time was qa. A game that relies on physics takes a lot of work to get right, and an open world makes it way more open to things that go wrong.

The time sink was probably in prototyping for new ideas to serve as the core of the game, then in generating content that would be considered innovative and fun for people to use that core with. Games are often a moving target where they need to try things that don’t work before finding ideas that will last.

I believe they also said they spent a year on final gameplay tweaks alone before releasing; TotK is a great example of why we shouldn't be mad when a game is delayed again in again