Is this on account of cost, or simplifying verification? I was under the impression that minimising the overall risk of a design was a major consideration. I'd love to know more about how these design decisions were made.

Power consumption and cost. 10 years on a battery is very hard to achieve for microcontroller.

https://sorwatch.net shows that : with much more advanced microcontroller (much more modern tech.node, low power features) - it only works for 1-2 years on a single battery after extensive power optimizations.

In custom silicon when seconds tick - it's maybe 100 transistors change their state. In microcontroller - it's roughly 10'000-100'000 transistors changing state to handle interrupt and update display content...