Of course. Every year there is usually a new area of focus for many subjects. This means building up a new library of content for that year's focus. Each year teachers will have a new cohort of students and what worked last year may not work this year. For example, there may be more students with specific learning needs who require content to be simpler, or on the flipside you may have a year of high flyers who need to be extended.

>I could assume once every few years a teacher somewhere has a breakthrough on how to teach something better

Education is it's own beast. It turns out that as a teacher you are constantly, every week learning and developing new and better ways of explaining content. Teaching is as much art as it is science. There is no settled "best way" to teach since we all learn at different pace and in different ways.

Teaching is also not only content. It's pacing and sequencing. As a teacher you're constantly learning what works and what doesn't just by applying your craft.

And finally schools move teachers around constantly. Last year you were teaching Year 8 History and this year you've been moved to Year 7 Social Studies. That requires a huge amount of effort to build up your own bank of activities, slideshows, links, videos etc.

It's a never ending task.