A designer I worked with years ago had a great explanation for why consistency is important. It’s not about a limited colour palette or a careful selection of fonts no one will ever notice. Chasing the specifics makes horrible software. Some people equate less diversity in their UI to more consistency. It’s about letting someone become an expert in your software. Microsoft office was always his example. People pride…
I believe you're right that expertise has something to do with it, but I don't think your comment quite connects why consistency leads to expertise.
Consistency is about taking what users already know about a system and compounding that expertise across other parts of the system. When a system is consistent, once I spend the effort to learn X and Y, then I also know Z and W because they'll behave the same way. In an inconsistent system, I can't extract as much value from my existing learning investment.